Transport Mystery

We started our first Transport Phenomena lab on the very first day of class. (Welcome to Olin.) To get a sense for fluid properties and some of the fluid instruments in the lab, we were given mystery fluids to identify based on our measurements of their surface tension, density, viscosity, thermal conductivity, and pH.

We split up into pairs, and each pair chose a fluid. My partner was Lise, who is also in Por Supuesto and on the Society of Women Engineers board with me. Before working with the mystery fluid, though, we practiced making measurements on water and compared our results to known values. After that, we moved on to the mystery fluid.


 The first thing we noticed once we started making measurements was that the fluid was really viscous and slimy. Trying to pipette the fluid from its jar into other containers was difficult because the mystery fluid stuck to everything. (Along those same lines, I had to change gloves so many times. So much sliminess.) The viscosity measurement, which involved a steel ball falling through a tube of the liquid, took over two minutes for each trial. For comparison, with water the measurement took about two seconds, and that was with a glass ball instead of a steel one.

One of the measurement tools we used was really awesome: the tensiometer, used to measure surface tension.

Tensiometer, used to measure surface tension of a fluid

There's a platinum-iridium ring that hangs on a hook on the left side of the picture. You fill the petri dish (sitting on a plate on the left side) with the fluid, and then raise the dish until the ring is submerged in the fluid. The hook for the ring is on an arm that needs to align with a small black line at all times during the measurement. To make the measurement, you turn two knobs at the same time. One knob lowers the petri dish with the fluid, and the other increases the force on the arm where the ring is hung. You turn both knobs very slowly, keeping the arm aligned with the black line, and then at some point the ring snaps free of the fluid, and the Vernier scale should read the fluid's surface tension.

It took a couple of times using the tensiometer to get used to it and to make the measurement well, but it's a cool instrument and surprisingly accurate.

Our mystery fluid was glycerin! We guessed that pretty early on, given the high viscosity of our fluid, and that guess was validated when we checked published numbers. Our measurements for density, surface tension, and thermal conductivity were within a percent for standard values for glycerin between 20 and 25 degrees C, which was awesome. Our viscosity measurement was a bit low, but a small amount of water and the air bubbles in the viscometer could account for that.