Sunday, September 03, 2017

Capillary action - the hidden foe in the physics of floods

There is an enormous amount of physics involved in storms and floods.   The underlying, emergent properties of water are key to much of this.

An individual water molecule can move around, and it can vibrate and rotate in various ways, but it's not inherently wet.  Only when zillions of water molecules get together does something like "wetness" of water even take on meaning.  The zillions of molecules are very egalitarian:  They explore all possible microscopic arrangements (including how they're distributed in space and how they're moving) that are compatible with their circumstances (e.g., sitting at a particular temperature and pressure).  Sometimes the most arrangements correspond to the water molecules being close together as a liquid - the water molecules are weakly attracted to each other if they get close together; at other temperatures and pressures, the most arrangements correspond to the water molecules being spread out as a gas.    Big tropical systems are basically heat engines, powered by the temperature difference between the surface layers of seawater and the upper atmosphere.  That difference in temperatures leads to net evaporation, driving water into the gas phase (by the gigaton, in the case of Hurricane Harvey).  Up in the cold atmosphere, the water condenses again into droplets, and heating the air.  If those droplets are small enough, the forces from adjacent air molecules bouncing off the droplets slow the droplets to the point where they are borne aloft by large-scale breezes - that's why clouds don't fall down even though they're made of water droplets.

There is another feature that comes from the attraction between water molecules and each other, and the attraction between water molecules and their surroundings.   Because of the intramolecular attraction, water molecules would have less energy if they were close together, and therefore having a water-air interface costs energy.  One result is surface tension - the tendency for liquid droplets to pull into small blobs that minimize their (liquid/vapor interface) surface area.

However, sometimes the attractive interaction between a water molecule and some surface can be even stronger than the interaction between the water molecule and other water molecules.  When that happens, a water droplet on such a surface will spread out instead of "beading up".  The surface is said to be hydrophilic.  See here.  This is why some surfaces "like" to get wet, like your dirty car windshield.

Sneaking in here is actually the hidden foe that is known all too well to those who have ever dealt with flooding.  You've seen it daily, even if you've never consciously thought about it.  It's capillary action.  A network of skinny pores or very high surface area hydrophilic material can wick up water like crazy.  Again, the water is just exploring all possible microscopic arrangements, and it so happens that in a high surface area, hydrophilic environment, many many arrangements involve the water being spread out as much as possible on that surface.  This can be to our advantage sometimes - it helps get water to the top of trees, and it makes paper towels work well for drying hands.  However, it can also cause even a couple of cm of floodwater indoors to ruin the bottom meter of sheetrock, or bring water up through several cm of insulation into wood floors, or transport water meters up carpeted stairs.   Perhaps it will one day be economically and environmentally feasible to make superhydrophobic wall and flooring material, but we're not there yet.

(To all my Houston readers, I hope you came through the storm ok!  My garage had 0.8m of water, which killed my cars, but the house is otherwise fine, and the university + lab did very well.)


Andrew Tan said...

Good to hear you came through the floods ok.

Jenny Li said...

Hi, Doug, glad to see your updates here, is everything OK?

Douglas Natelson said...

Hi Jenny, yes - the lab is good, my family is fine. While we lost our old cars, we were actually very fortunate relative to many.