Manipulation and separation of objects at the microscale, in solution and at interfaces

Abstract

Many separation techniques rely on different physical or chemical characteristics of the objects being separated. This includes separations based on size, total charge, or strength of interaction with a substrate. Recently there are many contexts in which it is important to manipulate or separate objects with much more subtle differences. For example, there has been significant interest in separating cells with different deformabilities because disease states can lead to changes in flexibility or stiffness, as observed in the red blood cells in sickle cell anemia. Proteins are another example in which manipulating molecules based on their flexibility or deformability (e.g. due to unfolding or di-sulfide bonds) is currently challenging. Further, genomic-length DNA separation and manipulation has direct applications in the development of novel DNA mapping and sequencing devices.

We have developed a computational model that can efficiently simulate the dynamics of rigid as well as flexible objects in a combination of electric field gradients and pressure driven flow, and have used this model to show that conformation-dependent electrophoretic mobility can be used to trap and manipulate objects. We are interested in using the predictions from the simulations to design microfluidic devices to trap and manipulate deformable objects. The interplay between the electric field and the fluid flow in the microchannels, given the coupled dynamics of the effective field on these objects and their conformation, can allow for manipulation and stretching in a unique way. We have extended the technique to perform simulations of the manipulation and trapping of rod-like objects such as Tobacco Mosaic Virus (TMV). Apart from looking at objects in solution, we have examined the structure and dynamics of polymers at aqueous interfaces through Brownian dynamics simulations.

With these broad guides in mind, the overarching goal of our work is to gain a better understanding of the manipulation and separation of objects at the microscale, both in solution as well as at an interface. The key principle underlying most of our work is the conformation-dependent electrophoretic mobility of the object; as the object changes its conformation, the mobility changes, which leads to a different electrophoretic velocity and response to electric field gradients.

Apart from manipulating objects in solution in the cases above, it is also interesting to study the self-assembly of objects at interfaces. Interface mediated self-assembly of practical interest includes many solutes interacting to form aggregates with internal structures (e.g., fibrils), coatings (e.g., films of heteropolymers or unfolded proteins), and other larger structures of technological interest forming over larger time-scales.