Monday, September 10, 2012

I probably should have posted this earlier, but if you are in downtown fargo tomorrow with nothing to do...

Wednesday, August 15, 2012

Weekend Fun


The fuel pump went on my truck last week (a 1998 F150 4.6L V8).  So I had to drop the tank and install a new pump.  The pump failed because it relies on rubber components in order to maintain pressure in the fuel lines.  This might sound surprising; the pump in this vehicle is located inside the gas tank where we might expect it to be relatively protected.  The failure here falls under the mysterious category of ‘ageing’.  It is not always clear what happens to the elastomer exposed to gasoline, but the result is clear.  The stress eventually leads to fracture, and failure of the component.  Add it to the list of other good problems in polymer science!

Saturday, July 21, 2012

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.


Saturday, March 17, 2012

Aspiration of a Block Copolymer Emulsion Droplet

In this movie we see an oil droplet (in water) that is loaded with a surfactant being drawn into a thin micropipette.  In particular we use poly ethylene oxide - b - poly styrene as a surfactant, and toluene as the oil.  As the drop is drawn into the pipette, its surface area changes and with a measurement of the pressure applied by the pipette a force - surface area curve can be constructed.  Remember your first year physics - a force vs distance graph will get you a spring constant!  In other words, we are measuring the surface modulus of the droplet.  We hope that fooling around with the surfactant molecule concentration etc. will lead us to a better understanding of the molecular details of the interface.

Wednesday, March 7, 2012

Buckled Particle Films

Here are two Laser Scanning Confocal Microscope images of a buckled monolayer of polystyrene particles. What should be clear is that there is a repeating pattern to the buckles (a wavelength). This is in remarkable contrast to what is expected - conventional elastic instabilities of this sort (known as wrinkles) require a bending modulus in the film. A traditional bending modulus cannot occur in the particle film, as the particles are free to rotate. The physics is about the 'torque chain', the way each sphere's rotation is matched by its neighbours. We are working on a very simple model, but are struggling with the difference between a well ordered hexagonally packed crystal of monodisperse spheres, and the disorder in the polydisperse sample.

Polydisperse Spheres


Monday, January 9, 2012

Converting from the Dimension 3100 to real units

Instructions:
This works for dimension 3100 software 0x0531000 anyway...

Open your afm data in imageJ. Draw a line and extract a cross section (i.e. Control K). Copy and paste the data to a spreadsheet. The intensity of the image is considered a z-coordinate now. Next we need to convert the x and z columns to realspace measurements. The x conversion is easiest: open the AFM file in a text editor (e.g. notepad). Then locate the image resolution and size (for example the image is 256 pixels by 256 pixels and the scan size is 40 000 nm). Then multiply each value by 40000/256 (the x scale factor) to convert to nm (the real space units).

The Z scale is conceptually the same, but slightly more complex to do. Again we wish to make a scale factor to convert our data. to do so we must locate the following lines in the header:
\@Sens. Zscan: V 14.30724 nm/V
\@2:Z scale: V [Sens. Zscan] (0.006713765 V/LSB) 334.2918 V
Note the numbers at the end will be different in each image, they are what we need to make the image specific conversion factor. In this case the conversion is 14.30724 x 334.2918 / 65535. The first two numbers are from the file (nm per volt and volts per digital value) and the last is the number of digital levels available to the scanner. Multiply your z column by this number and you are all set.