Chile-Chicago HOME

January 2006

Simon Poblete

1/27/06 Simon's project aims to create ordered arrays of magnetic nanoparticles. Working with Thu Tranh(?) in Prof. Jaeger's lab, Simon has learned the technology of making massively ordered gold nanoparticle arrays at free liquid surfaces developed in the group. He's learned Thu's technique of transferring the delicate monolayer to an electron microscope grid for observation and has good results in preserving the lattice order. Now comes the hard part. They want to precipitate cobalt nanospheres on the gold lattice so that the cobalt spheres adopt the ordering of the gold ones. So far they see the cobalt on the surface but have only achieved heterogeneous deposition. They have lots of tricks left to try.

6/26/06 Simon then made arrays with both gold and cobalt in a commensurate lattice. He also made gold arrays with a periodic pattern of rectangular holes, each containing an array of 20-30 cobalt particles. These arrays of nonmagnetic and magnetic particles will be useful in the group's future studies of the effect of magnetic scattering on the electronic transport through these nanodot arrays


Carlos Orellana

Carlos worked with Nataliya Yufa in Prof. Steven SibenerŐs group on aligning micron-sized particles in hard-walled rectangular channels. It is known that nanometer-sized particles spontaneously align in troughs of channels upon solution casting. Carlos observed and recorded the real-time alignment of microparticles following simple drop casting. To create alignment, he learned the technique of optical lithography and produced appropriately-sized channels. He then varied the size and density of the microparticles to eliminate electrostatic and gravitational interactions. At the end of his stay, he obtained video of rows of microparticles aligning in channels, overcoming many experimental challenges in a short span of time.

David Chen

6/26/06 Hau Ho in Prof. Rice's group is developing a new microscopic probe to follow correlated motions in a dense monolayer of colloidal particles. Light scattered from the particles makes moving interference patterns as the particles move. By measuring the fluctuating intensity of light emerging in different directions, they can infer the degree of collective motion at different length scales. This measurement requires precise alignment that has not yet been achieved. David made two adjustable holders for the fibers to align them at the center of the microscope stage and tested its alignment. He was also making a sample cell for the colloidal particles to use in his epxeriments. David's contributions in making these holders were greatly appreciated. He will be acknowledged in publications coming from this experiment.

Sebastian Michea

2/2/06 Sebastian is building devices for Prof. Cheng Chin's laser atom trapping lab. The laser beams have to be controlled and monitored. Their frequency has to be precisely controlled. this control is effected by measuring the amount of light on a photodiode. Then the diode signal is put through some op-amp circuitry and the output goes to a controller. The laser is tuned to an atomic absorption line, so its intensity is an accurate reflection of the frequency relative to the absorption frequency.

6/26 Sebastian made one amplified photo-detector and did a careful characterization of its performance and optimized its bandwidth and sensitivity. This photodetector forms part of an experiment that Prof. Chen is building to study ultracold fermionic matter.

Octavio Albarran

Octavio made a PID temperature controller for liquid krypton 83. The controller is for a new detector for hypotheical axion particles that Prof. Juan Collar is building. It requires liquid Krypton 83. Krypton is only liquid in a 5 degree range around 120 degrees K. Thus nontrivial temperature control and active heating are required to maintain the temperature. The control system must be sophisticated in order to achieve the best stability. The "PID" type of controller anticipates the rate of cooling and heating and responds accordingly. The setup consists of a temperature transducer and a valve in a nitrogen line leading leading to a cooling jacket. Both are computer controlled from Labview. Octavio built and tested the electronics that runs the valve. He then interfaced the valve to a computer running labview software. He implemented the PID algorithm that monitors the rate of temperature drift. After later adjustments by others, the controller is now operational.

Maite Cerda

Cell membranes and membranes inside of cells have to merge and divide at certain times but resist this merging and division at other times. One element thought to control the merging and dividing tendency is the presence of short sequences of the same amino acids that make up proteins. These molecules known as peptides can attach to cell membranes and alter their tendency to merge or divide. Maite is studying this tendency in the lab of Chemistry professor Ka Yee Lee. She is working with a certain class of peptides and a certain class of simplified cell membranes called lipid vesicles. She senses the attachment by the microscopic amount of heat generated when a peptide attaches to a vesicle. She can also monitor the speed of the attachment and the influence of the peptide concentration.

Sebastian Reyes

2/15/06 Sebastian came to ask Tom Witten about how to think about capillary vs viscous forces on droplets in a microfluidic channel. Prof. Rustem Ismagilov wants to separate successive aqueous droplets with a buffer droplet that is immiscible with both the aqueous fluid droplets and the oily carrier fluid.

6/27/06 Sebastian assisted graduate student Delai Chen in deducing what conditions on interfacial tension are needed to achieve the desired buffer effect. He made inerfacial tension measurements on various candidate liquids and helped Delai in demonstrating their ability to separate the aqueous drops in real microchannels. Delai has drafted a paper reporting these findings, of which Sebastian will be a co-author.

Fluid channel containing a sequence of four droplets. The the second and fourth one are being separated by the third one, demonstrating the desired buffer effect. ---Courtesy of Delai Chen.

German Varas

German is studying sand jets in Prof. Nagel's lab. He shoots a stream of fine glass particles against a flat target and watches the patterns that form using a fast camera. The jets are surprisingly different from individual particles. Individual particles bounce nearly elastically from the surface. However, the stream of particles forms a pancake in the plane of the target. With larger, heavier particles the plan begins to turn into a cone pointing downstream. Ripples form both radial and tangential on the length scale of the thickness of the pancake. The morphology of shapes with target shape and particle size and view angle are very rich. The coherence of the pattern seems to be due to the mutual entrainment of the particles by air. German wants to assess the role of air by varying the available parameters.

nail target When the jet hits a flat target that is smaller than the jet, the jet deflects to form a cone of a well-defined angle. This angle can be determined by assuming a) the target absorbs momentum of the jet in proportion to its area, and b) the kinetic energy (and thus the speed) of the drop is unaffected by the collision. These two facts together imply a linear relationship between the target diameter and the deflection angle. when the target is small. The same facts explain the analogous behavior of a liquid at the highest velocities where surface energies are least important. Some discrepancies are still seen. These may be due to incomplete local equilibrium or air drag.