Tuesday, June 17, 2008 - 2:20 PM
Room 8b (McKimmon Conference Center)

Modeling Anisotropic Particle Behavior within Optical Landscapes in Microfluidic Systems

Brandon L. Conover and Michael J. Escuti. North Carolina State University, Raleigh, NC

Highly selective manipulation of nano- and micro-scale particles within microfluidic systems is of great interest to the medical and scientific communities as well as commercial institutions. Predictable control of colloidal particles allows for the design and construction of myriad microfluidic processes, e.g., mixers, sorters, pumps, and entire lab-on-a-chip (LOC) systems for such complex routines as DNA analysis and high-throughput screening of cell populations.

Experimental work by our group and others suggests means of selective, non-invasive, and parallel control over particles of anisotropic shape (including blood cells, nanorods, etc.). The behavior of these particles in microfluidic systems is much less understood than the ubiquitous sphere. In a departure from the well-known and electrically-controlled dielectrophoretic approaches, however, our work considers optical tweezers and optical interference landscapes as the tools with which to control our particles of interest.

Although experiments show manipulation effects that are compelling and repeatable, there exists no complete analytical description of the phenomena encompassing anisotropic particles, arbitrary intensity gradients, and the effects of polarizability. Our recently published model approaches this goal in its ability to mathematically and analytically obtain the complete motion of spheroidal dielectric particles within optical interference landscapes under laminar flow, i.e., within microfluidic environments. Primarily, our model reveals the activity (translation and rotation) of anisotropic particles based on optical power, environment boundaries, suspension media, flow velocity, and other parameters relevant to the system in question.

In an effort to ensure the accuracy of our model, we have developed a second based on light scattering, specifically the T-matrix method. This technique has been widely used for determining light scattering properties by non-spherical, mesoscale particles. By analyzing the incident and scattered fields, our second model extracts the activity of the particle(s) via optically-induced forces and torques.

Here we briefly describe the salient features of both models, focusing on the utility and comparative simplicity of our first. In summary, we aim for the development of a complete and consistent modeling scheme for determining the behavior of dielectric, polarizable, mesoscale particles of anisotropic shape in arbitrary intensity gradients. An accurate but simple model facilitates prediction of particle response in this context, that should promote lower prototyping and production costs, improved and more efficient microfluidic device functionality, and a greater understanding of the behavior of colloidal systems within optical fields.