Claudio visited from about January 10 to about March 10, 2004. His project was supervised by Terry Bigioni in the laboratory of Prof. Heinrich Jaeger. Claudio's project aims to gain a deeper understanding of a colloidal crystallization phenomenon that we have been using to make nanoparticle arrays. Under the proper conditions, these arrays show massive order. Circumstantial evidence suggests that the ordering is aided by kinetic conditions. The project consisted in reproducing the crystallization using large colloidal particles. These particles move more slowly than the nanoparticles and are visible in a light microscope. This makes it easy to both enhance and observe the kinetic effects. Our specific aim was to observe the "crushing effect" carrying particles to the free surface during evaporation.
Claudio began is experiments immediately soon saw many phenomena that occur when a drop of colloidal suspension dries. After brief explorations of many such phenomena, Claudio focused on one demonstration. Using a very dilute dispersion of 1 micron weakly charged silica spheres, he monitored the number of particles confined to the surface during very slow evaporation and very rapid evaporation. In rapid evaporation the free surface recedes much faster than the particles can move by Brownian motion. (The diffusion length at the velocity of the free surface is smaller than the particle diameter.) Claudio observed a large increase in the number of surface particles after 30 seconds of rapid evaporation as compared to slow evaporation to the same final volume. One expects the number of surface particles to equal the number originally in the evaporated liquid volume. The observed number, though large, is quite small compared to this expected number. We have two hypothesis to account for this. The first is that there is some surface potential barrier that prevents particles which are pushed towards the surface to actually reach it. The second is that the strong transverse flows observed during rapid evaporation carry particles away from the observed surface.
The strong evaporation carrys with it many ancillary effects. The evaporation is done by putting the sample in a dessicator and evacuating it. This leads to strong convection, likely due to surface tension gradients. It also leads to rapid cooling with ultimate formation of a fog of ice crystals. The ancillary effects have to be sorted out, in order to interpret Claudio's findings clearly.
In the last days of his stay, Claudio repeated these experiments in a different geometry. The small droplet was replaced by a cylindrical well of water. This change of geometry affected the result strongly. The number of particles at the surface came much closer to the expected number.
Claudio wrote a 10 page report summarizing his results to date. He presented his work at a Jaeger group meeting. Both the report and the presentation were clear, interesting, useful, and well done. Claudio returned to Chile with the necessary materials to continue his work there. It was anticipated in the program proposal that projects begun during the exchange would continue when the student returned. With the colloid samples we provided and the microscopes already available in his lab, Claudio expects to be able to continue his studies. The program is supposed to facilitate communication so that projects can continue to completion after students return. Claudio plans to meet with us remotely every week. We expect this project to lead to a scientific publication, hopefully with Claudio as the principal author.
Claudio, Witten, Jaeger and Bigioni expressed great satisfaction with the this exchange to date. A suitable standard of comparison is our ongoing program of Research Experiences for Undergraduates, held June-August every year. These REU projects almost always produce a report that is useful for continuing the project with future students. Occasionally it grows into a prominent science paper in which the REU student plays a major role. Claudio's visit to date has produced a report and a body of research that are comparable to the best REU work. Moreover, the work is on track to become a useful and maybe prominent paper.
4/4/04 Gustavo has just returned to Chile. He has laid the groundwork for a clear and understandable computation of the drag on a long droplet in a square channel in very slow movement. He is writing up his findings now. In the lab, Gustavo discovered some striking secondary structure that appears when the drop moves rapidly through the channel. It appears that pockets of the carrier fluid appear next to the flat walls near the head of the drop and which move with it. Each pocket appears to leave a trail of little droplets that stay fixed on the walls. Gustavo says he is hooked on this problem and intends to keep pursuing it.
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