MaterialsInMotion

The focus of the day was all about surface tension. In the morning, Professor Kolodka defined surface tension as force per unit length, and it’s the tendency for the molecules of a liquid to contract and minimize its surface area. This concept explains why bubbles are spherical: spheres have the largest volume-to-surface-area ratio. Surface energy, on the other hand, is energy per unit area, but dimensional analysis gives you the same units as surface tension. The difference is that surface energy is a more general term that also applies to solids. Professor Kolodka then explained how polarity affects surface energy. Polarity increases surface energy because it allows molecules to form strong bonds with each other.

Our lab in the afternoon was led by our cluster’s third professor, Roberto Andresen Eguiluz, who is from Mexico City and leads the research team that the PhD students helping us in the lab are a part of. Our lab consisted of using a micropipette to dispense 20 microliters of water onto substrates with different treatments, such as Rain X, sanding, and repeated touching with barehands, and observing how the water behaved once it landed on the substrates. While I expected the water to bead up on the substrates treated with Rain X and repeated touching (since our hands are naturally oily), I was surprised to find that the water immediately puddled on the sanded substrate. A quick Google search taught me that high surface roughness increases surface energy because the surface area available for molecular interaction is increased. Since sanding the substrate degrades the substrate and makes it rougher, its surface energy was increased, making it more hydrophilic because the water, which also has a high surface energy, is more easily able to form intermolecular bonds with the sanded substrate.

The part of the day I found the most interesting was when Professor Kolodka introduced colloids, which are a type of solution in which microscopic particles are suspended evenly throughout another substance. He explained that soap was a colloid of water and surfactants (meaning surface active agents), which have a hydrophilic head group and a hydrophobic tail similar to the structure of phospholipids. Apparently, soap doesn’t actually kill germs. Instead, when you lather your hands with soap, the surfactants in the soap strip the germs, bacteria, and viruses of their phospholipid bilayers, which makes them unable to interact with other cells and infect you with sickness.

Another interesting thing I learned was that cars are coated with wax, which is hydrophobic, to cause water from rainfall to bead up and skid off the car. This behavior allows the car to remain cleaner and shinier for longer, and it also protects the paint underneath. However, I wonder why wax is the chosen coating. Why not another material? Is wax already the most optimized one?