Скачать или смотреть Holiday Physics: Why Do We STILL Not Understand Ice Skating? (And How Does... Ketchup Come In?)

Rheology, Thixotropic Fluids and Ketchup! The perfect recipe for a holiday special!... Right?






Further Discussion of Apparatus:

The device used in the PRX paper was called a “stroke-probe” and it was literally basically a fancy tuning fork with a glass bead glued to it and this glass bead would be touched ever so gently to the surface of ice and thus play the role of our skate top layer. And this tuning fork could be driven into motion at a certain precise oscillating frequency… because ya know… again, it's a tuning fork. And as it oscillates back and forth it basically skates across this thin layer of meltwater. And because a tuning fork is such a high quality oscillator, by having a kind of feedback bit of electrical circuity that actively varies the driving force oscillating the fork in order to keep the resonant frequency constant, they can take that data (necessary change in driving force needed to maintain the resonant frequency) they get a measure of the physical drag on the end of the bead and thus a measure of VISCOSITY. Also by varying the driving force and thus amplitude of the oscillation (i.e. how far it swings back and forth) while maintaining the same oscillation frequency they could vary the speed of the glass bead as it skates and thus get a measure of shear rate under different conditions.

But this wasn't the end of it. Tuning forks actually have MULTIPLE resonances and overtones, and especially when you consider that they can oscillate not just in the transverse (i.e. left-right) direction, but also the normal (i.e. up-down) direction. And exploiting this, they used a second oscillator driver to drive the tuning fork at a different resonance in this direction simultaneously. Though only slightly. Where in the transverse direction the fork was moving back and forth with an amplitude of about 10 microns, in the up-down direction it was only moving about 50 NANOmeters, so it's moving up and down about a thousand times less than it's moving left to right.

And the reason they did this is because the FRICTION felt between two objects is proportional to the amount they're pushed together, the so-called normal force. Ya know you can push a book across a table very easily but if someone pushes down real hard on it, it gets a lot harder because the normal force has increased. And the PROPORTIONALITY between a given normal force and frictional force that results is called the coefficient of friction, it's basically the innate slipperiness of the material.

So using this kinda brilliantly simple apparatus, they could basically measure both the viscosity and coefficient of friction of melt water as a function of shear rate. And they could actually do this with NANOMETER precision, that's precision to billionths of a meter and even estimate the THICKNESS of this layer, even though the tuning fork itself was about a centimeter or so large.