Fluidic wrench: Precise control over the position and orientation of anisotropic colloids using fluid flow

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. In this work, we demonstrate simultaneous control over the 2D center-of-mass position and orientation of anisotropic colloidal particles using only fluid flow. We use a 4-channel microfluidic device with a model-predictive control scheme to generate a flow pattern 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 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. Moving forward, this approach could be further engineered to achieve fluidic-directed assembly of asymmetric objects on a meso- to micro-scale level.