Sunday, July 23, 2006

This week in cond-mat

Just two papers this time. For the first, I must make a disclaimer: this is certainly not my area of expertise, and I can't really judge the validity of the results, but the topic is very interesting. I also haven't read either of these in any detail - they just look intriguing.

cond-mat/0607492 - Joly et al., Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics.
I vividly remember a great APS meeting talk by Seth Putterman (I think 10 years ago at the big centennial meeting in Atlanta) on basic pieces of table-top physics that we still don't really understand. One that he mentioned was triboelectricity - the separation of charge due to some frictional process. Remember junior high when you were told to rub a lucite rod with rabbit fur to build up a static charge? Amazingly, we still don't really understand the microscopics of this (unless the situation has changed recently. Any enterprising readers out there know anything about this?). Anyway, this paper is about the fluid analog of this. When a fluid containing ions is placed in contact with the walls of a container, the ion distribution is altered. Depending on the microscopic details of the fluid and the wall material, a sub-monolayer of charge can become practially immobilized at the wall (the Stern layer), and beyond that there extends into the fluid a net charge density (decaying exponentially into the fluid on a scale called the Debye length) set by competition between charge screening and diffusion due to concentration gradients (the appropriate diff-eq is the Poisson-Boltzmann equation). All this stuff is very important when worrying about colloidal suspensions, net charge on nanoparticles in solution, electrochemical scanned probe, etc. When fluid is flowing, slippage of the fluid layer right next to the wall can strongly modify the ion concentrations, and this can have big consequences for electrokinetic processes like electro-osmosis and electrophoresis. That's what this paper is on, and it's directly relevant to lots of micro- and nanofluidics work going on, particularly in the lab-on-a-chip community.

cond-mat/0607354 - Qi and Flatte, Current-induced spin polarization in nonmagnetic semiconductor junctions

Kato et al. showed recently that it's possible to build up a net spin polarization in the carriers in a strained nonmagnetic semiconductor (e.g. GaAs) by applying an electric field (and hence driving current into one side of the semiconductor through a junction, and out the other side). Lots of questions were inspired by this - is this a spin-orbit effect? Is this a spin-Hall effect? Now this new paper argues that the effect is neither of these things, and happens even in the absence of spin-orbit effects and for purely spin-independent scattering mechanisms. The trick seems to be that the mobility of carriers ends up depending nontrivially on the spin polarization (see here) for reasons that I don't currently understand. Seems profound enough that I should try to learn about it, though.

1 comment:

Incoherent Ponderer said...

Re: electrostatic charging due to friction - apparently it is still unresolved. George Whitesides of Harvard gave a talk recently where he used this as one of the examples of unresolved questions. He argued that just because we "know" Schroedinger equation, doesn't really help us in explaining a lot of things about seemingly simple mundane everyday physics. And if we cannot directly solve Schroedinger equation for anything more complex than Hydrogen atom (and even there we run into difficulties - just read history of calculations of what pressures H2 becomes metallic) - do we actually gain anything from knowledge that cannot be applied practically?

I think it's actually a rather important point that experimental or at least practical-minded scientists should remind string theorists.