The physics of drying your hands

We've all been there:  You wash your hands after using the restroom facilities, and turn away from the sink only to find one of those sad, completely ineffectual, old-style hot-air hand dryers bolted to the wall.  You know, the kind with the info graphic shown to the right (image credit:  nyulocal.com).  Why do these things work so poorly compared to paper towels?  What insight did Excel and Dyson have that makes their systems so much better?

It all comes down to the physics of trying to dry your hands.  At a rough estimate, the surface area of your hands is around 430 cm².   If your hands, when wet, are coated on average by a layer of water 100 microns thick (seems not crazy), that's a total volume of water of 4.3 cm³.  How can you get that water off of you?  One approach, apparently the one pursued by the original hot air dryers, is to convert that water into vapor.  Clearly the idea is not to do this by raising the temperature of your hands to the boiling point of water.  Rather, the idea is to flow hot, dry air over your hands, with the idea that the water molecules in question will acquire the necessary latent heat of vaporization (the energy input required to pull water molecules out of the condensed (liquid) phase and into the vapor phase) from their surroundings - the dry air, your hands, etc.  This "borrowing" of energy is the principle behind evaporative cooling, why you feel cold when you step out of the shower.

[A digression in fancy thermodynamic language:  When liquid water is in contact with dry air, the chemical potential for the water molecules is much higher in the liquid than in the air.  While the water molecules are attracted to each other via hydrogen bonds and polar interactions, there are so many many more ways that the water molecules could be arranged if they were diluted out into vapor in the air that they will tend to leave the liquid, provided each molecule can, through a thermal fluctuation of some sort, acquire enough energy to sever its bonds from the liquid.  The departing molecules leave behind a liquid with a lower average total energy, cooling it.  Note that water molecules can come from the vapor phase and land in the liquid, too, depositing that same latent heat per molecule back into the liquid.  When the departure and arrival processes balance, the vapor is said to be at the "saturated vapor pressure", and evaporative cooling stops.  This is why sweating a whole bunch on a super humid day does not cool you off.]

Back to your hands.  Converting 4.3 cm³ of water into vapor requires about 9700 Joules of energy.  If you wanted to do this with the heat supplied by the hot air dryer, and to do it in about a minute (which is far longer than most people are willing to stand there rubbing their hands as some feeble fan wheezes along), the dryer would have to be imparting about 160 W of power into the water.  Clearly that's not happening - you just can't get that much power into the water without cooking your hands!  Instead, you give up in disgust and wipe your hands discreetly on your pants.

In contrast, paper towels use thermodynamics much more effectively.  Rather than trying to convert the water to vapor, paper towels take great advantage of (1) the very large surface area of paper towels, and (2) capillary forces, the fact that the liquid-solid surface interaction between water and paper towel fibers is so attractive that it's energetically favorable for the water to spread out (even at the cost of increasing more liquid-vapor interface) and coat the fibers, soaking into the towel.  [Bonus physics lesson:  the wet paper towel looks darker because the optical properties of the water layer disfavor the scattering processes on micron-scale bits of fluff that tend to make the towel look white-ish.]  Yes, it takes energy to make paper towels, and yes, they must then be disposed.  However, they actually get your hands dry!

What about Excel and Dyson?  They realized very clearly that trying to vaporize the water on your hands is a fool's errand.  Instead, they try to use actual momentum transfer from the air to the water to blow the water off your hands.  Basically they accelerate a stream of air up to relatively high velocity (400 miles per hour, allegedly, though that sounds high to me).  That air, through its viscosity, transfers momentum to the water and that shear force drives the water off your hands.  They seem to have found a happy regime where they can blow the water off your hands in 10-15 seconds without the force from the air hurting you.    The awesome spectacle of those good dryers just shows how sad and lame the bad ones are by comparison.

Cleanrooms - what is new and exciting?

Cleanrooms - basically climate-controlled, dust-mitigated environments filled with equipment useful for micro/nanoscale fabrication and associated characterization - are a staple of modern research universities.  What kind of tool set and facilities you need depends on what you're trying to do.  For example, if you want to teach/do research on the fabrication of high performance Si transistors or large-scale integrated circuits, you probably want a dedicated facility that deals primarily with Si CMOS processing.  That might include large-area photolithography or wafer-scale e-beam lithography or nanoimprint lithography tools, evaporators/sputtering systems/PECVD/RIE/ALD systems able to service 150 mm or 200 mm substrates, and you might want to keep non-Si-friendly metals like Au far far away.  On the flip side, if you are more interested in supporting microfluidics or MEMS work, you might be more interested in smaller substrates but diverse materials, and tools like deep etchers and critical point dryers.

We're about to embark on a cleanroom upgrade at my institution, and I would appreciate input from my relevant readers:  What in your view is the latest and greatest in micro/nanofab tools?  What can't you do without?  Any particularly clever arrangements of facilities/  Assume we are already going to have the obvious stuff, and that we're not trying to create a production line that can handle 200 mm substrates.  Conversely, if you have suggestions of particular tools to avoid, that would also be very helpful.  Insights would be greatly appreciated.

Brief items, public science outreach edition

Here are a couple of interesting things I've come across in terms of public science outreach lately:

• I generally f-ing love "I f-ing love science" - they reach a truly impressive number of people, and they usually do a good job of conveying why science itself (beyond just particular results) is fun.  That being said, I've started to notice lately that in the physics and astro stories they run they sometimes either use inaccurate/hype-y headlines or report what is basically a press release completely uncritically.  For instance, while it fires the mind of science fiction fans everywhere, I don't think it's actually good that IFLS decided to highlight a paper from the relatively obscure journal Phys. Lett. B and claim in a headline that the LHC could detect extra spatial dimensions by making mini black holes.  Sure.  And SETI might detect a signal next week.  What are the odds that this will actually take place?  Similarly, the headline "Spacetime foam discovery proves Einstein right" implies that someone has actually observed signatures of spacetime foam.  In fact, the story is the exact opposite:  Observations of photons from gamma ray bursts have shown no evidence of "foaminess" of spacetime, meaning that general relativity (without any exotic quantumness) can explain the results.   A little improved quality control on the selection and headlines particularly on the high energy/astro stories would be great, thanks.
• There was an article in the most recent APS News that got me interested in Alan Alda's efforts at Stony Brook on communicating science to the public.  Alda, who hosted Scientific American Frontiers and played Feynman on Broadway, has dedicated a large part of his time in recent years to the cause of trying to spread the word to the general public about what science is, how it works, how it often involves compelling narratives, and how it is in many ways a pinnacle of human achievement.  He is a fan of "challenge" contests, where participants are invited to submit a 300-word non-jargony explanation of some concept or phenomenon (e.g., "What is a flame?", "What is sleep?").  This is really hard to do well!  
• Vox has an article that isn't surprising at all:  Uncritical, hype-filled reporting of medical studies leads to news articles that give conflicting information to the public, and contributes to a growing sense among the lay-people that science is untrustworthy or a matter of opinion.  Sigh.
• Occasionally deficit-hawk politicians realize that science research can benefit them by, e.g., curing cancer.  If only they thought that basic research itself was valuable.

"Flip chip" approach to nanoelectronics

Most people who aren't experts in the field don't really appreciate how amazing our electronic device capabilities are in integrated circuits.  Every time some lithographic patterning, materials deposition, or etching step is performed on an electrically interesting substrate (e.g., a Si chip), there is some amount of chemical damage or modification to the underlying material.  In the Si industry, we have gotten extremely good over the last five decades at either minimizing that collateral damage, or making sure that we can reverse its effects.  However, other systems have proven more problematic.  Any surface processing on GaAs-based structures tends to reduce the mobility of charge in underlying devices, and increases the apparent disorder in the material.  For more complex oxides like the cuprate or pnictide superconductors, even air exposure under ambient conditions (let alone much lithographic processing) can alter the surface oxygen content, affecting the properties of the underlying material.

However, for both basic science and technological motivations, we sometimes want to apply electrodes on small scales onto materials where damage from traditional patterning methods is unavoidable and can have severe consequences for the resulting measurements.  For example, this work used electrodes patterned onto PDMS, a soft silicone rubber.  The elastomer-supported electrodes were then laminated (reversibly!) onto the surface of a single crystal of rubrene, a small molecule organic semiconductor.  Conventional lithography onto such a fragile van der Waals crystal is basically impossible, but with this approach the investigators were able to make nice transistor devices to study intrinsic charge transport in the material.  

One issue with PDMS as a substrate is that it is very squishy with a large thermal expansion coefficient.  Sometimes that can be useful (read this - it's very clever), but it means that it's very difficult to put truly nanoscale electrodes onto PDMS and have them survive without distortion, wrinkling, cracking of metal layers, etc.  PDMS also really can't be used at temperatures much below ambient.  A more rigid substrate that is really flat would be great, with the idea that one could do sophisticated fab of electrode patterns, and then "flip" the electrode substrate into contact with the material of interest, which could remain untouched or unblemished by lithographic processes.

In this recent preprint, a collaboration between the Gervais group at McGill and the CINT at Sandia, the investigators used a rigid sapphire (Al2O3) substrate to support patterned Au electrodes separated by a sub-micron gap. They then flipped this onto completely unpatterned (except for large Ohmic contacts far away) GaAs/AlGaAs heterostructures.  With this arrangement, cleverly designed to remain in intimate contact even when the device is cooled to sub-Kelvin temperatures, they are able to make a quantum point contact while in principle maintaining the highest possible charge mobility of the underlying semiconductor.  It's very cool, though making truly intimate contact between two rigid substrates over mm-scale areas is very challenging - the surfaces have to be very clean, and very flat!  This configuration, while not implementable for too many device designs, is nonetheless of great potential use for expanding the kinds of materials we can probe with nanoscale electrode arrangements.

Tunneling two-level systems in solids: Direct measurements

Back in the ancient mists of time, I did my doctoral work studying tunneling two-level systems (TLS) in disordered solids.  What do these words mean?  First, read this post from 2009.   TLS are little, localized excitations that were conjectured to exist in disordered materials.  Imagine a little double-welled potential, like this image from W. A. Phillips, Rep. Prog. Phys. 50 (1987) 1657-1708.

The low temperature thermal, acoustic, and dielectric properties of glasses, for example, appear to be dominated by these little suckers, and because of the disordered nature of those materials, they come in all sorts of flavors - some with high barriers in the middle, some with low barriers; some with nearly symmetric wells, some with very asymmetric wells.   These TLS also "couple to strain" (that's how they talk to lattice vibrations and influence thermal and acoustic properties), meaning that if you stretch or squish the material, you raise one well and lower the other by an amount proportional to the stretching or squishing.

When I was a grad student, there were a tiny number of experiments that attempted to examine individual TLS, but in most disordered materials they could only be probed indirectly.   Fast forward 20 years.  It turns out that superconducting structures developed for quantum computing can be extremely sensitive to the presence of TLS, which typically exist in the glassy metal oxide layers used as tunnel barriers or at the surfaces of the superconductors.  A very cool new paper on the arxiv shows this extremely clearly.  If you look at Figure 2d, they are able to track the energy splittings of the TLS while straining the material (!), and they can actually see direct evidence of TLS talking coherently to each other.  There are "avoided crossings" between TLS levels, meaning that occasionally you end up with TLS pairs that are close enough to each other that energy can slosh coherently back and forth between them.   I find this level of information very impressive, and the TLS case continues to be an impressive example of theorists concocting a model based on comparatively scant information, and then experimentalists validating it well beyond the original expectations.   From the quantum computing perspective, though, these little entities are not a good thing, and demonstrate a maxim I formulated as a grad student:  "TLSs are everywhere, and they're evil."

(On the quantitative side:  If the energy difference between the bottoms of the two wells is \(\Delta\), and the tunneling matrix element that would allow transitions between the two wells is \(\Delta_{0}\), then a very simple calculation says that the energy difference between the ground state of this system and the first excited state is given by \(\sqrt{\Delta^{2} + \Delta_{0}^{2}}\).  If coupling to strain linearly tunes \(\Delta\), then that energy splitting should trace out a shape just like the curves seen in Fig. 2d of the paper.)

Table-top particle physics

We had a great colloquium here today by Dave DeMille from Yale University.   He spoke about his group's collaborative measurements (working with John Doyle and Gerry Gabrielse at Harvard) trying to measure the electric dipole moment of the electron.  When we teach students, we explain that as far as we have been able to determine, an electron is a truly pointlike particle (infinitesimal in size) with charge -e and spin 1/2.  That is, it has no internal structure (though somehow it contains intrinsic angular momentum, but that is a story for another day), and that means that attempts to probe the charge distribution of the electron (e.g., scattering measurements) indicate that its charge is distributed in a spherically symmetric way.

We know, though, that from the standpoint of quantum field theory like quantum electrodynamics that we should actually think of the electron as being surrounded by a cloud of "virtual" particles of various sorts.   In Feynman-like language, when an electron goes from here to there, we need to consider not just the direct path, but also the quantum amplitudes for paths with intermediate states (that could be classically forbidden), like spitting out and reabsorbing a photon between here and there.   Those paths give rise to important, measurable consequences, like the Lamb shift, so we know that they're real.  Where things get very interesting is when you wonder about more complicated corrections involving particles that break time reversal symmetry (like B and K mesons).  If you throw in what we know from the Standard Model of particle physics, those corrections lead to the conclusion that there actually should be a non-zero electric dipole moment of the electron.  That is, along its axis of "spin", there should be a slight deficit of negative charge at the north pole and excess of negative charge at the south pole, corresponding to a shift of the charge of the electron by about \(10^{-40}) cm.  That is far too small to measure.

However, suppose that there are more funky particles out there (e.g., dark matter candidates like the supersymmetric particles that many people predict should be seen at the LHC or larger colliders).  If those particles have masses on the TeV scale (that'd be convenient), there is then an expectation that there should be a detectable electric dipole moment.  DeMille and collaborators have used extremely clever atomic physics techniques involving optical measurements on beams of ThO molecules in magnetic and electric fields to look, and they've pushed the bound on any such moment (pdf) to levels that already eliminate many candidate theories.

Two comments.  First, this talk confirmed for me once again that you really have to have a special kind of personality to do truly precision measurements.  The laundry list of systematic error sources that they considered is amazing, as are the control experiments.  Second, I love this kind of thing, using "table-top" experiments (for certain definitions of "table") to get at particle physics questions.   Note that the entire cost of the whole experiment over several years so far as been around $2M.  That's not even a rounding error on the LHC budget.  Sustained investing at a decent level in this kind of work may have enormous bang-for-the-buck compared with building ever-larger colliders.

March Meeting, days 1 and 2

I am sufficiently buried in work, it's been difficult to come up with my annual March Meeting blog reports.  Here is a very brief list of some cool things I've seen:

• Jen Dionne from Stanford showed a very neat combination of tomography and cathodoluminescence, using a TEM with tilt capability to map out the plasmon modes of individual asymmetric "nanocup" particles (polystyrene core, gold off-center shell).
• Shilei Zhang presented what looks to me like a very clever idea, a "magnetic abacus" memory, that uses the spin Hall effect in a clever readout scheme as well as a spin transfer torque way to flip bits.
• I've seen a couple of talks about using interesting planar structures for optical purposes.  Harry Atwater spoke about using plasmons in graphene to make tunable resonant elements for, e.g., photodetection and modified emissivity (tuning black body radiation!).  My former Bell Labs department head Federico Capasso spoke about using designer dielectric resonator arrays to make "metasurface" optical elements (basically optical phased arrays) to do wild things like achromatic beam steering.
• Chris Adami had possibly the most ambitious title, "The Evolutionary Path Toward Sentient Robots".  Spoiler:  we are far from having to worry about this.
• Michael Coey spoke about magnetism at interfaces, including a weird result in CeO2 nanoparticles that appears to have its origins in giant orbital paramagnetism.
• There was a neat talk by Ricardo Ruiz from HGST about the amazing nanofabrication required for future hard disk storage.  Patterned media (with 10 nm half-pitch of individual magnetic islands) looks like it's on the way.

There were a number of other very nice talks.  It's pretty clear that I could have spent the whole meeting so far at "beyond graphene" 2d materials talks and things to do with spin-orbit coupling.  A bit more tomorrow.

Brief items + the March APS Meeting

This has been an absurdly busy period for the last few weeks; hence my lower rate of posting.  I hope that this will resolve itself relatively soon, but you never know.  I am going to the first three days of the March APS meeting, and will try to blog about what I see and learn there, as I have in past years.

In the meantime, a handful of items that have cropped up:

• If you go to the APS meeting, you can swing by the Cambridge University Press table, and pre-order my nano textbook for a mere $64.  It's more than 600 pages with color figures - that's a pretty good deal.  They will have a couple of bound proof copies, so you can see what it looks like, to a good approximation.  If you teach a senior undergrad or first-year grad sequence on this stuff and think you might have an interest in trying this out as a text, please drop me an email and I can see about getting you a copy.  (My editor tells me that the best way to boost readership of the book is to get a decent number of [hopefully positive] reviews on Amazon....)
• On a related note, you should really swing by the Cambridge table to order yourself a copy of the 19-years-in-the-making third edition of Horowitz and Hill's Art of Electronics.  I haven't seen it yet, but I have every reason to think that it's going to be absolutely fantastic.  Seriously, from the experimental physics side, this is a huge deal.
• This is a fun video, showing a "motor" made from an alkaline battery, a couple of metal-coated rare-earth magnets, and a coil of uninsulated wire.  It's not that crazy to see broadly how it works (think inhomogeneous fields from a finite solenoid + large magnetic moment), but it's cool nonetheless.
• Here's an article (pdf) that's very much worth reading about the importance of government funding of basic research.  It was favorably referenced here by that (sarcasm mode = on) notorious socialist organization (/sarcasm), the American Enterprise Institute.

Centers, institutes, and all that

One hallmark of the modern research university is the proliferation of Centers and Institutes, groupings of investigators outside the hierarchy of traditional academic departments.  I'd like to explain a bit about what these entities are, what (in my view) makes an effective one, and some challenges that these organizations face.  There is a great deal of variance across universities with these terms; the version I'm going to describe is mostly what we have at my home institution.  Your mileage may vary, and I'd like to hear your thoughts on what makes a great center or institute.

An Institute is an organization that draws members from across different departments (indeed, often from across different Schools such as Natural Science and Engineering), with a strong, usually broad, thematic focus, and with an annual budget for staff and programs that comes largely from internal university funds.  Institutes support programs that benefit their membership.  Examples of programs include:  seminar series; topical workshops and conferences, including interaction with companies, political entities, or the media; visitor programs; educational forays such as interdisciplinary graduate programs, training grants, research experience for undergraduates or teachers, K12 outreach days; endowed postdoctoral fellowships; etc.  An Institute is meant to act as a a catalyst or enabling structure to bring together researchers with a common intellectual interest, to foment new and support existing collaborations, and to further research activity in that area.  At some universities, an Institute may have its own building or administer shared infrastructure.

A Center is usually a smaller, more focused group of researchers that is often expected to be financially self-supporting through multi-investigator external funding.  (Sometimes these are called Laboratories.)   Centers often exist within or are founded as the result of institutes.  A Center likely concentrates on a portfolio of specific research-related projects, rather than having broad programmatic efforts like an Institute.  The US NSF sponsors a number of center programs (MRSECs, ERCs, STCs, CCIs, and formerly NSECs), as does the US DOE (EFRCs).

An effective Center is almost self-defining:  It is able to accomplish focused research goals and to raise sufficient external resources to be self-supporting at least on the several year to decade timescale.  A good Center is able to identify and adapt to new research opportunities, while realizing which avenues are becoming played-out and should be set aside - basically, effective self-criticism while encouraging creativity through seed projects to generate new activity.   Longevity and research productivity metrics are two ways to assess the utility of a Center.

An effective Institute needs to serve its members by successfully supporting and carrying out its programs.  Because many Institutes have very broad programmatic goals, this requires serious "buy-in" - a decent fraction of the membership have to be willing to invest their time and energies to ensure that these programs are a success.  This only happens when the people involved really believe in the efforts and can see that they and the institution derive real benefit from the work.   This means that the Institute has to be responsive to the needs of its membership.   At the same time, the university has to assess (via research and funding metrics) the impact of the Institute and its programs, since the university has to decide whether the internal resources of the Institute could have been better spent elsewhere.

Both Institutes and Centers can be vulnerable to budgetary problems (internal and external, respectively) and to lack of engagement by membership.  At most places (the University of Chicago seems to be a big exception, since there Institutes have a lot of power) an Institute can be particularly exposed in tight times, since departments and much of their budgets are explicitly necessary for the reaching mission of the university, while Institutes are often viewed as elective or discretionary expenses.  In terms of engagement with members, like many organizations, Institutes and Centers succeed by succeeding and fail by failing.  You can't force people to collaborate, but once some do arise, productive collaborations lead to further productive collaborations.  Overall, Centers and Institutes appear to be key components of successful research universities.  It's not clear how these organizational structures (and their associated programs) will fare if we are in a long era of declining federal funding and internal cost cutting.

2d metallicity at low temperatures - a nice new result

To quote this blog from about 8.5 years ago (!): 

For years now, there has been a fairly heated debate about the nature of an apparent metal-insulator transition (as a function of carrier density) seen in various 2d electronic and hole systems. The basic observation, originally made in some Si MOSFETs of impressively high interface quality made in Russia, is that as the 2d carrier density is reduced, the temperature dependence of the sheet resistance changes qualitatively, from a metallic dependence (lower T = lower resistance) at high carrier concentration to an insulating dependence (lower T = higher resistance) at low concentration, with a separatrix in between with nearly T-independent resistance at some critical carrier density. A famous 1979 paper by the "Gang of Four" (Anderson, Abrahams, Licciardello, and Ramakrishnan) on the scaling theory of localization had previously argued that 2d systems of noninteracting carriers all become insulating at T=0 for arbitrarily weak disorder.

So, there has been a long-simmering controversy about why some 2d systems (electrons or holes) seem to show a really metallic temperature dependence of their conductance at low temperatures.  This dependence, where the conductivity apparently increases by, say, a factor of 2 from \(T =\) 4.2K down to 0.1 K, takes place over a temperature range where the scattering of electrons by lattice vibrations (the mechanism responsible for the increase in conductivity of ordinary metals as they are cooled from room temperature down to cryogenic temperatures) is supposed to be all finished.   I mentioned this as an ongoing controversy in '06 and again in '12.  What is going on here?

There is a new preprint from Bruce Kane and colleagues at Maryland that clarifies things considerably, in my view.  Kane, probably best known for proposing a quantum computing scheme involving individual phosphorus donors in Si, is a very clever experimentalist.  He has developed a method of creating field-effect transistors,where the conducting channel is the hydrogen-terminated surface of a Si wafer, and the gate dielectric is vacuum (!).  Using these devices, his group has been able to look at the apparent metallicity in both electrons and holes in the same system.  They find that the improvement in conduction at low temperatures has to do with the screening of charged impurities by the conducting system (and for the experts:  in Si the electrons are able to do this better than the holes because there are 6 conduction band valleys, while there is no valley degeneracy for the holes).  This doesn't directly get to the "fundamental" question about whether the true, zero-temperature ground state is insulating in a real, interacting system, but it does go a long way toward demonstrating why the conductivity still has a metallic change with temperature even though phonons should be out of the picture.

Updated: Advice on choosing a grad school

Over the last week I've run into a couple of readers of this blog who pointed out that many people never find older posts (unless they happen to use google with just the right search terms), and that it might be valuable to re-run updated versions of some of those, particularly the ones geared toward career advice.  This makes lots of sense, given how long this blog has been running and how readership has evolved.  So, here is the first of these updated re-runs (from 2011):  Advice on choosing a grad school. 

This is written on the assumption that you have decided, after careful consideration, that you want to get an advanced degree (in physics, though much of this applies to any other science or engineering discipline).  This might mean that you are thinking about going into academia, or it might mean that you realize such a degree will help prepare you for a higher paying technical job outside academia.  Either way,  I'm not trying to argue the merits of a graduate degree.

• It's ok at the applicant stage not to know exactly what you want to do.  While some prospective grad students are completely sure of their interests, that's more the exception than the rule.
• If you get the opportunity to visit a school, you should go.  A visit gives you a chance to see a place, get a subconscious sense of the environment (a "gut" reaction), and most importantly, an opportunity to talk to current graduate students.  Always talk to current graduate students if you get the chance - they're the ones who really know the score.  A professor should always be able to make their work sound interesting, but grad students can tell you what a place is really like.
• I know that picking an advisor and thesis area are major decisions, but it's important to realize that those decisions do not define you for the whole rest of your career.  I would guess (and if someone had real numbers on this, please post a comment) that the very large majority of science and engineering PhDs end up spending most of their careers working on topics and problems distinct from their theses.  Your eventual employer is most likely going to be paying for your ability to think critically, structure big problems into manageable smaller ones, and knowing how to do research, rather than the particular detailed technical knowledge from your doctoral thesis.  A personal anecdote:  I did my graduate work on the ultralow temperature properties of amorphous insulators.  I no longer work at ultralow temperatures, and I don't study glasses either; nonetheless, I learned a huge amount in grad school about the process of research that I apply all the time.
• Always go someplace where there is more than one faculty member with whom you might want to work.  Even if you are 100% certain that you want to work with Prof. Smith, and that the feeling is mutual, you never know what could happen, in terms of money, circumstances, etc.  Moreover, in grad school you will learn a lot from your fellow students and other faculty.  An institution with many interesting things happening will be a more stimulating intellectual environment, and that's not a small issue.
• You should not go to grad school because you're not sure what else to do with yourself.  You should not go into research if you will only be satisfied by a Nobel Prize.  In both of those cases, you are likely to be unhappy during grad school.  
• I know grad student stipends are low, believe me.  However, it's a bad idea to make a grad school decision based on a financial difference of a few hundred or a thousand dollars a year.  Different places have vastly different costs of living - look into this.  Stanford's stipends are profoundly affected by the cost of housing near Palo Alto and are not an expression of generosity.  Pick a place for the right reasons.
• Likewise, while everyone wants a pleasant environment, picking a grad school largely based on the weather is silly.
• Pursue external fellowships if given the opportunity.  It's always nice to have your own money and not be tied strongly to the funding constraints of the faculty, if possible.  (It's been brought to my attention that at some public institutions the kind of health insurance you get can be complicated by such fellowships.  In general, I still think fellowships are very good if you can get them.)
• Be mindful of how departments and programs are run.  Is the program well organized?  What is a reasonable timetable for progress?  How are advisors selected, and when does that happen?  Who sets the stipends?  What are TA duties and expectations like?  Are there qualifying exams?  Where have graduates of that department gone after the degree?  Know what you're getting into!
• It's fine to try to communicate with professors at all stages of the process.  We'd much rather have you ask questions than the alternative.  If you don't get a quick response to an email, it's almost certainly due to busy-ness, and not a deeply meaningful decision by the faculty member.  For a sense of perspective:  I was traveling yesterday, and during that time my email queue expanded by about 50 messages, not counting all the obvious spam I deleted. 

There is no question that far more information is now available to would-be graduate students than at any time in the past.  Use it!  Look at departmental web pages, look at individual faculty member web pages.  Make an informed decision.  Good luck!

What are liquid crystals?

Once you accept the idea that the simple, microscopic interactions between bits of matter can lead to the emergence of dramatic collective properties when large numbers of particles are concerned, it's not surprising to realize that there are many different ways that large ensembles of particles end up organizing.  As mentioned previously, a true liquid is a system where the average distance between the particles is comparable to the particle size, but the particles are in constant motion and there is no particular long-range order to the way the particles are arranged.

New possibilities present themselves if the particles have some kind of "internal degree of freedom".  For example, think of the particles not as little featureless billiard balls, but as elongated objects.  Now we can consider having the orientation of all the particles have some long-range correlation.  A liquid crystal is an emergent phase when the particles are close together and there is not 3d spatial order in the arrangement of particle positions, but there is order in the orientations of the particles.  In nematic liquid crystals, the centers of mass of the particles are completely spatially disordered, but there is long-range order in their orientation. For example, they could all be pointing the same direction, indicated by the not-so-cleverly-named vector, the director.  Cholesteric liquid crystals have some twist or chirality to the particle orientation.  In smectic liquid crystals, the particle centers of mass are actually spatially ordered in one direction, but not in the other two (i.e., you can think of stacks of layers of particles, with particles free to move within each layer).  The wiki page about liquid crystals gets into the history of these systems, and here is a nice webpage that classifies them.  Liquid crystals are very useful because their directed nature gives them anisotropic optical properties, and if the objects in question are polar molecules, it is possible to reorient them electrically.  This combination enables many technologies, almost certainly including the display device you're using to read this.

There was a time when I was somehow skeptical that all these phases were "real" thermodynamic phases.  I was used to solids, liquids, and gases, and I'd learned about "hard" condensed matter phases like ferromagnets and superconductors that dealt with emergent properties of the electron gas.  Somehow these liquid crystal things didn't seem like the same sort of thing to me.  Then I read the really great book by Chaikin and Lubensky, and saw things like the figure at right (from G. S. Iannacchione and D. Finotello, Phys. Rev. E 50, 4780 (1994)).  The figure shows the specific heat of a liquid crystal (in some nanopores) as it goes through a thermally driven transition between the nematic and isotropic phases, as a function of scaled temperature, \(t \equiv (T/T_{\mathrm{c}})-1\).  This kind of sharp, divergent feature and scaling as a function of temperature are hallmarks that show these phases and their transitions are every bit as real as any other thermodynamic phase, even though the materials are squishy.

Java applets for physics - a great resource being strangled by security?

As many of my readers know, starting in the late '90s, many clever, creative people around the world wrote cute (and sometimes very sophisticated) Java applets to demonstrate certain physics and engineering concepts.  Examples include this great site by the University of Buffalo, a virtual lab by the University of Oregon, this resource by UCLA, this outstanding site from the University of Barcelona, etc.  Many of us owe a real debt of gratitude for these resources, as they have been great educational tools. 

A problem has arisen, however.  You will notice that none of the applets linked above actually run.  Because of security concerns about Java, the latest versions of Java require applets to have been compiled, authenticated, and certified (via electronic security certificates), or the applets simply won't be run by the virtual machine.   For actively maintained sites (such as the excellent "physlet" effort from Davidson), the authors and maintainers have thoughtfully recompiled and updated their code.  Others (the University of Colorado) have rewritten everything (!) in Flash or HTML5.  Unfortunately, these are the exceptions, and many other cool sites are orphaned, with clever code that can't be run.

If anyone knows a work-around (some kind of emulator that would run the code in a walled-off way?), please describe it in the comments.  It would be a real shame if the accumulated excellence of all those older sites was wiped out.  Thanks.

Several items, including interesting reading

• Celebrity scientist Lawrence Krauss has written an article (pdf) about whether celebrity scientists are good for society, and noting that celebrity \(\ne\) greatest scientific researcher, necessarily.  In response to the title ("Celebrity scientists:  Bad for science or good for society?") it's tempting to be snarky and respond "Why not both?". Note that this guy is conspicuously absent from the article.
• Hat tip to the Angry Physicist for pointing out this article about the US military academies.  I found it genuinely shocking.  I'd always had the impression growing up, based on anecdotes I guess, that West Point and Annapolis in particular were incredibly selective and could be very academically demanding.  The academy graduates I've met over the years had only reinforced this idea by being very impressive people.  I was very dismayed to read about the apparently low academic standards.
• I was dismayed by two NSF-related issues in the last week.  First, NSF has gotten increasingly rigid about enforcing minutia of their guidelines over the last couple of years.  This is particularly frustrating when combined with guidelines that are themselves ambiguous (e.g., saying that a preproposal must include certain items, but not saying whether other items like collaboration letters are desired, or worse, forbidden because adding extra material can be grounds for getting bounced without review), and then being hard to reach for clarification.  This is a further sign that they are understaffed and overwhelmed.
• Second, in the Major Research Instrumentation call, NSF no longer allows grant funds to pay for technical staff.  That means that an approach that had previously been extremely helpful (have NSF pay for 75% of a staff person the first year, 50% the second, and 25% the third, so that a university can taper in technical staff support over time) is no longer possible. 
•  An old friend of mine does an excellent podcast, and he spent some time talking with me - it was really fun.

What is a phase (of matter)?

Defining "a phase of matter" for a popular audience is a tricky business, with choices ranging from the overly simplistic and therefore vacuous (a collection of matter that has homogeneous, uniform, well-defined physical properties that are distinct from other such phases), to the very technical, to sophistry (like the famous definition of obscenity).

A critical ingredient missing from the simple definition above is the deep, profound point that phases of matter only make sense as emergent from the collective behavior of many constituents (the dynamics of which are often governed by simple rules).  A single water molecule is not a solid, a liquid, or a gas - it is just a single molecule, with a structure and some mechanical, electronic, and optical properties that can be calculated with pretty good accuracy through "ab initio" techniques like density functional theory and its relatives.  (Note:  Even doing that is bloody hard, given that ten electrons is actually a lot from the standpoint of quantum chemistry.)

However, if you take a collection of \(N\) water molecules and stick them in a box of a fixed volume \(V\), with a certain amount of kinetic energy \(E\), and let them bounce around and do their thing, interacting with each other via van der Waals and longer-ranged (dipolar, since water is a polar molecule) forces, something interesting will happen.  To avoid difficult conceptual issues about reversibility, let's imagine you have a whole bunch of boxes like this, all prepared with the same \(N, V\) and \(E\) but with the microscopic initial conditions like molecular positions and velocities scrambled.  (This is the "microcanonical ensemble", for experts.)  Wait an unspecified long while. What you will find is that as \(N\) increases from 1 to a large number, at some point you will start being able to classify the emergent, "coarse-grained" properties of these boxes.  For a sufficiently low \(E\), you will find that the vast majority of the boxes contain a blob of water molecules that have arranged themselves in a spatially ordered way, with spatially periodic positions and orientations.  There will be a few leftover molecules bouncing around, and the blob will have a certain amount of jiggling going on.  If you shook the box, you would see that the blob moves rigidly, exhibiting some resistance to deformation, though the molecules at the edges would move more easily, and would be constantly exchanging with the few leftover molecules bouncing around the rest of the box.  Somehow, the molecules in those boxes have spontaneously broken a bunch of symmetries (picking out spatial locations that exhibit some periodicity and rotational symmetry), and what we think of as "bulk" properties have emerged, like density, some kind of elastic modulus, a speed of sound, etc.  There is now some interface as well, between the solid and the mostly unoccupied void. 

For higher \(E\), you will probably find that the vast majority of boxes contain a blob of water molecules that are very close together, bumping into each other all the time, but tumbling around with no particular relative orientation.  This blob of water has an interface with the remaining "gas", and does not respond rigidly if it bumps into a wall of the box.  If you could look at all the molecules, you could add up how much energy it takes to expand the surface of that blob - this is proportional to the surface tension.

At still higher \(E\), you will find that the water molecules are roughly homogeneously distributed throughout each box, bumping into each other and the walls.  You could still think about an average density for this gas, and if you banged on the wall of the box to impart momentum to the molecules that happen to be hitting that wall, you could watch the propagation of a density wave (sound!) through the molecules.   In the really high \(E\) limit, the molecules decompose and the constituent atoms ionize - this is a plasma.

Each of these arrangements that you would find in a very large percentage of such imaginary boxes, with its emergence of well-defined "bulk" physical properties (including more subtle ones I haven't mentioned, like magnetic order or electrical conductivity) as \(N\) grows to a statistically large value, is a thermodynamic phase of matter.    Why are these the particular ones that occur?  Why do water molecules tend to form particular solid structures?  Why don't we see the spontaneous appearance of phases that look very different, like long 1-d chains of water molecules, for instance?  It's not at all obvious!  That's the fun of condensed matter physics:  The answer somehow lies in the microscopic properties of the molecules and their interactions - it's latent in there as soon as you have one molecule, but somehow cannot emerge and be realized except through the collective response of a large ensemble.  More soon.

More posting soon.

Sorry for the brief hiatus.  Many urgent tasks (e.g., proposal deadlines, prep for semester, grad admissions).  Suggestions for forthcoming topics are always appreciated.  Coming up in future installments:  "When does water become wet?", "What is a phase?", "What are liquid crystals?", and "Centers, Institutes, and all that".

Great elasticity demonstration

I've linked to Dustin Sandlin before.  He does a fantastic series of YouTube videos that are readily accessible by a lay audience and show why science is fun.  Here is his look, using ultrahighspeed video, at why dry spaghetti tends to break in at least three pieces when flexed from the ends.  As he says, this is something that Feynman himself couldn't readily unravel.  Watch the video before you scroll down and read my spoiler description of the mechanism.

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This video shows some great elasticity concepts that we generally don't teach in the undergrad physics curriculum.  Flexing the noodle puts the top in tension and the bottom in compression - if you assume simple elasticity (e.g., stress = (Young's modulus)(strain)), and you consider the resulting forces and torques on a little segment of the noodle, you can calculate the shape (e.g., vertical deflection and tilt angle as a function of position along the noodle) of the bent spaghetti, though you have to assume certain boundary conditions (what happens to the displacement and tilt at the ends).  As the flexing is increased, at some point along its length (more on this in a minute) the noodle fractures, because the local strain has exceeded the material strength of the pasta.  One way to think about this is that the boundary condition on one end of each piece of the noodle has now changed abruptly.  Each piece of noodle now starts changing shape, since the previous strained configuration isn't statically stable anymore.  That shape change, the elastic information that the boundary condition has changed, propagates away from the fracture point at the speed of transverse sound in spaghetti (probably a couple of km/s).  The result is a propagating kink in the noodle, the severity of the kink depending on the local curvature of the pre-fracture shape.  If that local strain again exceeds the critical threshold, the noodle will fracture again.  The fact that we need really high speed photography to see the order of breaking shouldn't be that surprising - the time interval between fractures should be the size of the noodle fragment (around 3 cm) divided by the speed of sound in the pasta (say 2000 m/s), or around 15 microseconds!  (If I was really serious, I'd go the other way and use the video record of the propagating kink to measure the speed of transverse sound in pasta.)

This problem is actually somewhat related to another mechanics question: why do falling chimneys tend to break into three pieces?  Again, treating the chimney as some kind of elastic beam clamped at the bottom but free at the top, one can find the (quasi static, because the time it takes sound to propagate in the chimney material is much shorter than the time for the chimney to fall) shape of the flexing chimney.  There are two local maxima in the strain, and that's where the chimney tends to break.  Note that the chimney case is quasi static, while the spaghetti case really involves the dynamics of the flexing noodle after fracture. 

The bottom line:  I want one of those cameras.

Lack of self-awareness, thy name is John Horgan.

I see that Scientific American is reorganizing its blogging efforts.  I hope it works out well for them.  Call me if you want someone to blog about condensed matter and nanoscale science.  I'd really enjoy talking to a wider audience and would, of course, tailor my style accordingly.

When looking at their site, though, I came upon this piece by John Horgan, about whom I have written previously.  This latest essay is meant to be advice for young science writers.  Because he is a smart person with great experience in science journalism, his basic advice does have some kernels of merit (be skeptical of claims of scientists; pay attention to who is talking about science and their possible agendas).  His other points strike me as odd or beside the point to varying degrees.  (e.g., scientists are people and therefore have a human context to their work, but claiming that the majority of US science is shaped by capitalism and militarism is just nutty; inequality, our screwed up healthcare system, and militarism are all distressing, but what does that have to do with talking about a large part of science?)

The very first point that Horgan makes got my attention, though, and nearly broke my irony-meter.  He writes (his emphasis): "Most scientific claims are bogus. Researchers competing for grants, fame, glory and tenure often—indeed usually–make exaggerated or false claims, which scientific journals and other media vying for readers eagerly disseminate."  While I recognize that there have been claims to this effect in recent years, I think it is pretty hilarious that Horgan can warn about this with a straight face.  This is the guy who vaulted onto the larger, international stage by writing a book called The End of Science back in 1996.  Yeah, that wasn't at all an exaggerated or false claim made with the intent of capturing as much media attention as possible.  Nope.

Long odds: Proposals and how we spend our time

We just completed the two-day kickoff symposium of the Rice Center for Quantum Materials.  It was a good meeting, and the concluding panel discussion ended up spending a fair bit of time talking about the public policy challenges facing basic research funding in the US (with some discussion of industry, but largely talking about government support).  Neal Lane is an impressive resource, and lately he and Norm Augustine have been making the rounds in Washington trying to persuade people that it's a dire mistake to let basic research support continue to languish for the foreseeable future.

Over the December/January timeframe, I'm spending time on several grant proposals.  Three of them have a priori odds of success (based on past years, dividing awards by the number of initial proposals) less than 5%.  Now, obviously longshots have their place - you can't win if you don't play, and there is no question that thinking, planning, and writing about your ideas has utility even if you don't end up getting that particular award.  Still, it seems like more and more programs are trending in this awful positive feedback direction (low percentage chance per program = have to write more grants = larger applicant pool = lower percentage chance).  Many of these are prestigious center and group programs that are greatly desired by universities as badges of success and sources of indirect costs, and by investigators as sources of longer term/not-single-investigator support.  When yields drift below 5%, it really does raise questions:  How should we be spending our time, one resource that we can never replenish?  Does this funding approach make sense?  When the number of potentially "conflicted" people (e.g., coauthors/collaborators over the last four years for every person affiliated with a big center grant) exceeds 1000 (!), who the heck is left to review these things that has any real expertise?

Science and sensationalism: The allure of superlatives and bogus metrics

I helped out a colleague of mine today, who was fact-checking a couple of sentences in a story that's going to come out in a large circulation magazine (that shall remain nameless).  The article is about graphene, and in draft form included a sentence along the lines of "Graphene is 1000x better at conducting electricity than copper."  That sounds great and exciting.  It's short, simple, and easy to remember.  Unfortunately, it's just not true unless accompanied by a huge asterisk that links to a page full of qualifications and disclaimers. 

The challenge:  Come up with a replacement that gets the main point across (graphene is a remarkable material) without being a gross distortion or dissolving into scientific jargon. 

My response:  "Graphene is an electrical conductor that rivals copper and silver, and is much lighter and stronger."  At least this is true (or moreso, anyway), though it's longer and doesn't have an easy-to-remember number in it. 

The search for a simple, one-sentence, exclamatory pronouncement can lead science journalists (and university public relations people) down a dangerous path.  Often really great science is simply more complicated than a sound-bite.  Moreover, the complications can be fascinating and important.  It takes a special journalist to recognize this.