Photos by sophomores Josefin Betsholtz, Nicolas Gomez, Meryl Gibbs and Emily Shorin were winners in the class photo competition in 1.060 Engineering Mechanics II.

“In deciding what to photograph, how to capture the phenomenon, and how best to explain it, the students begin to link their physical understanding of fluid mechanics — most of which exists before they set foot in the classroom — with the formal analytical tools that form the basis of the course,” says visiting Associate Professor Marco Ghisalberti of the University of Western Australia, who taught the course this spring. Ghisalberti, who received his S.M. in 2000 and Ph.D. in 2005 from MIT, and Professor Heidi Nepf judged the photos on three criteria: aesthetic appeal, uniqueness of the phenomenon, and articulation of the fluid mechanics.

Photo / Josefin Betsholtz. View image in a larger size.

First Place: Josefin Betsholtz

Dye Droplets at an Oil-Water Interface
This image shows the portion of a glass filled with water (bottom, higher density) and coconut oil (top, lower density), and the droplets of food dye that rest on the interface between the oil and the water. It illustrates the effects of surface tension, both at the oil-water interface and at the surface of the droplets. The droplets are supported by the surface tension at the oil-water interface (they are on the oil side of the interface and thus they will not mix with the water just yet). Interfacial tension at the droplet surfaces means that they take on a spherical shape that minimizes their surface area. When the droplets diffuse through the interface and enter the water (with which they are miscible), they burst. Just below the oil interface, the different colors have not diffused into each other yet, but they have on the bottom of the water layer (as indicated by the darker color).

Photo / Nicolas Gomez. View image in a larger size.

Second place: Nicolas Gomez

Smoke Ring
Smoke rings are possible through the use of toroidal vortices. A toroidal vortex occurs when a fast-moving parcel of fluid is injected into a stationary fluid. Different parameters, such as temperature, relative speed, and size of the moving fluid all affect the “crispness” of a smoke ring. Normally, a vortex is a parcel of fluid spinning around a linear axis, like a tornado or hurricane. In a toroidal vortex, the axis is still there, but it loops and closes on itself so that the vortex forms a donut shape. Thus the spinning air traps the smoke inside the vortex, forming a barrier with the surrounding, stationary fluid. This spinning flow decreases the friction between this parcel of air and the stationary air around it. Thus the ring can travel for long distances and remain intact, while other smoke trails blown out with it dissipate.

Photo / Meryl Gibbs. View image in a larger size.

Third place tie: Meryl Gibbs

Marangoni Flow
The phenomenon captured in the photograph is surfactant-induced flow. A surfactant is a substance that lowers the surface tension of other liquids. Gibbs accomplished this by taking milk in a dish and adding food coloring to track the movement of the milk. She then added Dawn dish soap, the surfactant, which reacts with the milk by breaking down the fat, creating localized changes in surface tension and creating the initial circular movement outward. The motion of the milk occurs due the cohesive bonds being broken, lowering the surface tension. The surrounding high tension forces the fluid to compensate in a way that will restore equilibrium.

Photo / Emily Shorin. View image in a larger size.

Third place tie: Emily Shorin

Rayleigh-Taylor Instability
When a denser fluid rests on top of another fluid with less density, the Rayleigh-Taylor instability arises. Since gravity is present in this process, both fluids attempt to reduce the combination of their potential energy by moving to a lower-energy elevation. If the boundary between these two fluids was completely horizontal, the denser liquid would not break the interface, but since small deviations occur the denser fluid ultimately breaks through. In my demonstration, I dropped comparatively dense food dye into less dense water and captured the image as the dye descended to the bottom of the cup. As the denser liquid descends through water, it forms long fingers with mushroom-like caps. Large areas of the less dense liquid separate these strands. The equations that determine the occurrence of this phenomenon are quite complex, but can be summarized in a few inequalities. For this theory to be applicable, the vertical and horizontal length scales of the less dense liquid must be much greater than the height of the mixing region which in turn must me much greater than the viscous and diffusion scales.