Chicago students in Chile 2008

UC students atop Andes left to right: Andrew Alexander, Daniel Citron, Prof. Enrique Cerda, Jonathan Cheng, Sam Oberdick.

Andrew Alexander

Andrew had finished his third year as a chemistry major before joining Roberto Bernal's biophysics group. He joined an experiment being developed to study how cells detect and respond to applied force. The experiment was made to detect the motion of a fibroblast cell attached to a rubber substrate when this substrate is deformed. By observing the birefringence of the substrate and the motion of the cell, one can infer the stress imposed on the cell as well as its response. Andrew calibrated the birefringent response of the rubber sheet to verify that it made the four-lobe pattern predicted by the theory. In initial trials with simple video equipment, they found partial consistency with the prediction. They made some initial trials with cells on the membrane, as they learned to created reproducible and healthy conditions for the cells during observation.

Prof. Bernal found Andrew's help useful in these preliminary scoping experiments. Andrew valued his lab experience and very much valued the overall experience of Chile.

Jonathan Cheng

Jonathan had completed his 3rd year as a physics major when he joined Prof. Francisco Vivanco's group for his internship. Jonathan worked with a new apparatus that exerts controlled sliding and compressive forces on a layer of transparent beads. As the layer is sheared, concentrated lines of compression build up and then decay. Jonathan's work was aimed at studying the effect of particle shape on the form of these concentrated structures. He was also interested in the effects of the speed of sliding motion on the structures. His main job was to photograph the moving particles as one varied the control parameters systematically: shape, confining presssure and sliding speed. The thickness of the layers rise and fall as force struts between the moving walls form and break. The group observed qualitative differences in these rises and falls with different particle shapes. With the extensive set of videos that Jonathan made, the group has the means to analyze these effects quantitatively.

Dan Citron

Dan has finished his 3rd year as a physics major. He was keen to do numerical simulations that were tied closely to experiments. His job with Prof. Rodrigo Soto in Uchile was of this nature. Soto had developed a simulation of a granular monolayer on a vertically oscillating floor, to complement the experiments of Prof. Nicolas Mujica. The simulation reproduces the coexistence of liquid and solid structure seen in the experiment. The focus of this internship was to study the effect of friction. This allows one to see effects that are difficult to reproduce experimentally. For example, it was anticipated that increasing friction would create an abrupt change in the spatial density correlations, going from highly compressible to weakly compressible. In practice the transition proved not to be so abrupt.

Dan wrote that he learned a great deal about programming and about comparing simulation data to a predicted functional form. His work in English conversation sessions was valued for his clear speaking style.

Sam Oberdick

Sam, is a physics major who did his internship after his 2nd year. He worked in the lab of Prof. Nicolas Mujica at the Univerisidad de Chile. With grad student and former intern Nicolas Rivas, he helped in rebuilding an apparatus to study turbulent flow. In this apparatus a cylindrical tank of water is stirred by rotating a cylindrical rod at it center. At high flow speeds, the surface develops a tilt, whose orientation rotates with time at a nontrivial rate. Precise measurements of the onset of the tilt were impeded by lack of sensitivity and by lack of rigidity of the apparatus. Sam's work was instrumental in improving the measurement of the tilt. They replaced the former resistive probe of the local depth with a new capacitative probe that was significantly more precise. In tandem with this, Sam developed a software module that allowed this capacitor to work in lock-in mode. The software was used for other control functions elsewhere in the laboratory.

Sam was pleased with his impact on the experiment and his supervisor was also pleased at the lasting benefits of Sam's work.

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