Fork in the road: patterned surfaces direct microcapsules to make a decision.

Using computational modeling, we simulate the motion of compliant microcapsules on patterned surfaces. The microcapsules, which consist of an elastic shell and an encapsulated fluid, model biological cells or polymeric particles. We focus on a surface that is decorated with a Y-shaped pattern. As compared to the stem of the Y, one branch is relatively soft, and the other branch is relatively sticky. The capsules are driven to move over this substrate by an imposed fluid flow. Upon reaching the junction point, we find that deformable capsules preferentially move onto the sticky branch and stiffer capsules move onto the soft branch. Thus, through their inherent interactions with the patterned domains, the microcapsules are driven to "make decisions" about their path along the surface. Such surface patterning provides a facile means of routing particular capsules to specified locations in microfluidic devices and can form a fundamental component in creating fluidic circuits where microcapsules carry out simple logic operations.