Fun With Cornstarch

Motivated by a recent NPR story (http://www.npr.org/2011/03/05/134268980/could-cornstarch-have-plugged-bps-oil-well ), Evan St. Claire and Ross Belgarde, two NATURE (http://www.ndepscor.nodak.edu/NATURE/index.html ) students  and I decided to explore the physics of shear thickening fluids.  The hypothesis proposed in the NPR story, was that a shear thickening fluid such as cornstarch and water could have been used to plug the flow of an oil spill (such as the recent BP oil well disaster).  Our thoughts were that this was a bit of a pipe dream; we didn’t believe that the properties of the shear thickening fluid would remain unchanged when mixed with crude oil.

As we didn’t have a cup of crude oil available in the lab, we decided to use dish soap as a substitute.  We reasoned that dish soap had some of the slippery, complex characteristics of oil that we wished to mimic.  Our experiment involved a home build vibration setup modeled after Merkt et al. (see http://prl.aps.org/abstract/PRL/v92/i18/e184501 ).  

Ross and Evan found that mixtures of cornstarch and water of mass ratios of about 0.75 would always show interesting ‘tower’ formation when driven at about 100 Hz (see the video’s from the Merkt paper http://www.youtube.com/watch?v=DrcShENMaoI ).  Remarkably, the addition of a few grams of dish soap completely killed the behaviour.  Our belief was that the soap caused a change in the fundamental fluid behaviour of the mixture.  To check, we used a cone and plate rheometer and measured the viscosity of all mixtures (shown in the figure).  This device measures the stress transmitted by a fluid from one rotating plate to another as a function of the ‘speed’ of rotation (the strain rate).  This allows us to measure the viscosity of each fluid (again see the figure).

Figure 1.  A cone and plate rheometer.  Our cornstarch is the  white goop in the middle of the two stainless steel rods.

The pure cornstarch and water mixture slows a clear shear thickening behaviour (upward curve as a function of strain rate).  In fact, it became so viscous that it began to slip along the plates just as a solid would.  For comparison we used a very low weight ratio cornstarch water solution (effectively water) which gives a very low and fairly flat curve.  We would expect that a purely Newtonian fluid would show no shear rate dependence.  When soap was added to the formerly shear thickening solution its behaviour completely changed.  Its viscosity dropped, but more importantly its slope changed from positive to negative – it changed from shear thickening to shear thinning!  Only at much higher strain rates did it start to recover shear thickening behaviour.  

Figure 2.  Viscosity as a function of strain rate for all our mixtures.  Note the shear thickening data cuts off below 1 Hz due to slip, and the dilute solution cuts off below 1 Hz as it drains out of the gap too quickly in this range.

We concluded that it is not obvious that a shear thickening fluid would be able to plug a broken oil well.  While our mixture does not necessarily mimic the actual well conditions, it does show how much the viscosity of the shear thickening mixture can change with the addition of a small amount of a third fluid.  To settle the oil well question our experiments would have to be conducted on the exact materials and the same speeds that would be present in the oil well.