(155i) Simultaneous Orientation and Position Control of Anisotropic Brownian Particles using a Stokes Trap

Suspensions of anisotropic colloidal particles are commonly encountered in a wide array of applications, including smart materials for targeted drug delivery and injectable materials for therapeutic applications. A grand challenge in the field of directed assembly is to precisely assemble chemically and structurally distinct anisotropic particles into functional hierarchical structures. Such complex assembly schemes will require precise control over both the position and orientation of individual rods. Despite recent progress, it remains challenging to control both the center-of-mass position and orientation of single and multiple rod-like Brownian colloidal particles for these applications. In this work, we demonstrate simultaneous control over the 2D center-of-mass position and orientation of anisotropic Brownian colloidal particles (polystyrene rod-like particles) using a Stokes trap. In this way, the orientation and position of single rod-like particles is controlled using the sole action of fluid flow. In particular, we use a 4-channel Stokes trap with a model-predictive control scheme as a feedback controller to solve a constrained optimization problem in real time. Using this approach, a flow pattern is generated that translates and rotates rod-like particles from their initial state to a final desired position and orientation. Unlike alternative techniques that exploit intrinsic material properties of rod-like particles (e.g. index of refraction, magnetic properties, surface charge) to control position and orientation, our method imposes no restrictions on the physical or chemical properties of the particles, and hence, can be used for rods of any material and size, assuming they can be imaged or detected. Moving forward, this approach could be further engineered to achieve fluidic-directed assembly of asymmetric objects on a meso- to micro-scale level.