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41832-B9
Transport and Sorting of Colloidal Suspensions and Emulsions in an Optical Lattice
For some time there have been available basic optical techniques used to manipulate one or two microscopic objects at a time, but our work has advanced the technical means for creating arrays of optical traps. The statistical nature of experiments utilizing optical forces – which involve samples that are in the diffusive limit, where Brownian motion is significant – suggests that there might be a significant benefit to simultaneously conducting an array of optical trap experiments, by creating independent sets of traps across the experimental field of view.
At the same time, the very issue of whether or not such an array of experiments may be treated independently points also towards an altogether different class of studies, aimed at either probing or exploiting the wide array of physical mechanisms that might serve to couple spatially separated components. For example, working in my lab at Illinois Wesleyan with collaborators from the University of Illinois, we have begun biological studies of how cell-cell signaling changes character when, instead of dealing with a pair of cells isolated from all others, one deals with an ensemble, as this changes the conditions required for quorum sensing. In such cases, the use of an array of optical traps can ensure well-controlled, reproducible ensembles for systematic studies. We studied the early stages of biofilm formation using various mutant strains of bacteria, with different biofilm-related genes deleted; here, optical trap arrays are used to ensure geometric consistency from ensemble to ensemble.
However, optical trap arrays determine not only the equilibrium structures that assemble, but also the dynamics of particles passing through the lattices: the flow of those particles that are influenced most by optical forces tends to be channeled along crystallographic directions in a periodic trap array (referred to as an optical lattice). In such a lattice, we have shown that the magnitude of the optical forces is an oscillatory function of particle size, and we have demonstrated that, as a consequence, it is possible to tune the lattice constant so as to make any given particle type either strongly interacting with the light or essentially non-interacting. This is of key importance for all-optical sorting of biological suspensions. Moreover, we noted that while the laser power delivered to the optical lattice may be, in total, significant, the intensity integrated over each biological cell can be a fraction of what is used in conventional optical tweezers, so all-optical sorting does relatively little to stress the extracted cells.
We have now published our work examining four classes of transport phenomena, involving the traffic of colloidal suspensions within, respectively, a static optical lattice with a DC fluid flow, a continuously translating lattice with a DC fluid flow, a flashing lattice with AC fluid flow, and a flashing lattice with combined AC and DC fluid flow. We found that continuous lattice translation helps to reduce nearest neighbor particle distances, providing promise for efficiency improvements in future high throughput applications, and unusual behaviors within flashing lattices that deserve further attention.
Our approach to sorting on an optical lattice extends previous methods for Holographic Optical Trapping, by moving the DOE to a point that is conjugate to the image plane, rather than the Fourier plane. Because of this conjugacy condition, the use of a fan-out DOE results in the convergence of multiple beams in the trapping volume, and the associated formation of an interference pattern. We have shown that this approach can produce high-quality 3D arrays of traps over a large region of space, which can be tuned in ways that include the lattice constant, the lattice envelope, and the degree of connectivity between trap sites.
During this third year of the PRF grant, I spent one month in France, visiting a lab with expertise in microemulsions. I also published three new papers. In addition, I have been Chair, as well as Editor of the Proceedings, with Kishan Dholakia, of the Optical Trapping and Micromanipulation Conference, sponsored by SPIE - The International Society for Optical Engineers, and expect to continue as co-chair. We have been able to solicit over 100 presentations from around the world for each of the conferences, (making these the largest conferences in the field). Also, Kishan and I have been offering a four-hour introduction to optical micromanipulation at the major U.S. conferences in optical engineering. This year, my invited research talks included:
81st American Chemical Society Colloids & Surface Science Symposium – June 24-27, 2007.
Advanced Technologies for Optical Micromanipulation, Cambridge, UK – May 9-10, 2007.
American Physical Society, March Meeting, Denver, CO – March 5-9, 2007.
Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland – Feb. 12, 2007. FSB Seminar
European Science Foundation Research Conference “Trends in Optical Micromanipulation,”
Alpine Center of Obergurgl in Tirol, Austria – Feb. 4-9, 2007.
Northwestern University – Sept. 28, 2006. Condensed Matter Seminar
