Leonardo Gordillo
Leonardo is studying a surprising phenomenon that happens when a stream or jet of granular particles collides with a target. The ejected particles also form a coherent stream if the incident jet is properly shaped. It is part of the PhD research of Xiang Cheng in Prof. Nagel's lab. The horizontal glass tube at lower left shoots the stream toward the target where Leonardo is pointing. The jet passes through a lens-shaped hole in the plastic mask shown in the closeup. Leonardo's mask has several lenses of various shapes but the same area. They will begin testing it with jets in a few days
update March 13 --- The expected anisotropic ejection was observed, but it was less dramatic than previous experiments with rectangular masks. Project will be continued with the plastic mask replaced by harder material and with measurements of the particles' velocities.
Suomi Ponce
Suomi is studying another form of granular stream, in free fall from a funnel. The brightly lit stream is shown in the lower-left part of the picture. Working with PhD student Scott Waitukaitis in Prof. Jaeger's lab, she has made a series of nozzles of varying sizes. The fast video camera at the bottom of the picture allows one to discern the motions of individual grains. Software analyzes the images and determines the velocities of the grains; then it distinguishes the shared streaming motion from the individual random motions. The amount of random motion is the granular temperature. It appears to vary with the distance fallen and with the diameter of the stream. Suomi's experiments will show these trends quantitatively.
update March 13 --- By cutting the circular hopper in half vertically and covering the cut with a sheet of plexiglas, Suomi could film the grains before they leave the hopper. It seems that the granular temperature rises to a maximum at the exit point and then decreases as the grains fall. This project is continuing.
Fernando Garcia
The swirling blue and black stripes at right are an atomic force microscope picture made by Fernando. It shows a thin film of special polymers, deposited in the clean room shown at left. Chemical interactions among these long, chainlike molecules leads to stripes with a width of tens of nanometers. Working with Stephanie Fronk in Prof. Sibener's lab, Fernando is exploring a new way to use these stripes to create metal wires at this tiny scale. Fernando will first deposit a thin, uniform layer of gold and chromium on top of the polymer film. Then they will try various novel approaches to remove the metal resting on the stripes but not the metal between stripes, thus creating stripes of metal.
update March 13 --- He studied the persistence of the stripes after rubbing out a region of the stripe pattern with the AFM tip. When the rubbed out region was small, the stripes grew back. They wanted to see how big the rubbed out region was necessary so that the stripes would not be able to grow back as they were. Project is continuing.
Desiree Salas
The familiar phenomenon of wrinkling of a thin skin takes on new forms when new materials are used. Desiree is studying thin films made with gold nanoparticles only a few nanometers across. Even three monolayers on the surface of water form a tough skin. Compressing the opposite sides of the skin creates wrinkles whose width indicates the bending stiffness of the skin. The white plastic water trough visible under the microscope objective at the left allows the wrinkles to be viewed. Under the supervision of student Brian Leahy in Dr. Binhua Lin's lab, Desiree is finding that ethanol added to the water makes a major change in the form of the wrinkles. She is studying how this change comes about as one gradually adds more ethanol. The added ethanol reduces the surface tension and may also have other effects on the film.
update March 13 --- They made a quantitative study with varying amounts of glycerol, ethanol and heat on the folding of the trilayers. They monitored the fold amplitude and found that it was generally comparable to the precursor wrinkle wavelength. They found conditions that gave a new buckling morphology unlike the long folds seen normally. Project is continuing.
Cecilia Gallardo
The microscope at the left is shrouded in plastic to keep the sample within clean. The microscope views a dollar-bill sized trough of water containing a single layer of 2 nanometer long lipid molecules on its surface. This particular monolayer folds in a distinctive way when the film is laterally compressed. It forms an array of folds slanted left and right at a well-defined angle. Working with Kseniya Gavrilov in Prof. Lee's lab, Cecilia capturing the folding motion on video tape. She is investigating conditions that affect the morphology of this folding, such as heating or the addition of glycerol to the water subphase. Understanding these effects will someday enable controlled folding of these monolayers, so that they can expand as needed to cover variable surface areas.
update March 13 --- Cecilia's DMPE lipid monolayers show deformation under lateral compression as seen previously for two-component lipids, A new feature is distinctive pattern of canted folds with a characteristic cant angle. At high temperatures, this folding behavior gives way to a smooth, nonbuckling response with novel patterns of localized protrusion into the subphase fluid. Project is continuing.
Rafael Otaiza
The stain left behind when a drop of dirty water dries on a surface is a robust way of shaping materials. The laws that control the distinctive density profile of stains like the one at right are known in general, but their practical limitations have not been worked out. Rafael is working with Prof. Witten to understand one important aspect. What is the mathematical law that governs the fading of the stain density as one moves towards the center? Rafael will study the effect of the drop's shape by numerically solving the governing flow equations. He will start by investigating elliptical shapes.
update March 13 --- Rafael analysed the previously derived equations to determine the drying behavior at the final stages of drying. The equations don't explain the uniform coating observed experimentally. He devised a numerical method to determine the fluid flow in an elliptical drop. He is continuing this work back in Chile.