Recent advances in the ease of fabricating particles with controlled shape have fostered excitement about the range of materials that can be created from assemblies of such particles. While attention has focused primarily on the influence of shape on the manner in which particles assemble due to random thermal motion, shape can also have surprising consequences for structure induced by fluid flows. Until recently, it had been thought that all axisymmetric, rigid particles rotated continuously in shear flows of viscous fluids. However, the investigators recently predicted the existence of a class of non-rotating ring-shaped particles with sharp outer edges. This project will include the application of computational and theoretical techniques to expand the range of flow-aligning particle shapes and explore their utility. An experimental program will explore methods to fabricate the particles and observe their flow aligning behavior. The fundamental studies will be guided by various target applications for materials with specific mechanical, rheological or permeability properties. The students involved in the research will be encouraged to explore these applications as inventor-scientists. Undergraduate researchers will aid with theoretical studies of chaotic dynamics and with the development of fabrication methods. The visually appealing nature of the research will be exploited in K-12 outreach activities at a local children?s science museum, the Ithaca Sciencenter.
The behavior of rigid particles that align in a low Reynolds number simple shear flow will be explored using boundary element method (BEM) computations and slender-body theory analysis. BEM simulations will be used to: (1) Search for cross-sectional shapes that are most effective in stopping rotation at moderate aspect ratios; (2) Explore the possibility that a ring with a filled center (to impart a gas barrier) can flow align; and (3) Show that rings with both in-out and fore-aft asymmetry exhibit a steady cross-streamline drift in their flow-aligned state. Slender-body theory (SBT) will be developed for rings with a large ratio of ring diameter to thickness. A novel aspect of this SBT is the need to determine the force per unit circumference induced by the velocity gradient near the ring cross-section. This cross term has not appeared in previous SBT for particles with circular or elliptical cross-sections. SBT simulations will determine whether flow-aligning rings can be induced to flip by hydrodynamic interactions with other rings and to determine the equilibrium position a particle with cross-stream drift reaches as it approaches a wall. SBT calculations will also be used to explore the possibility that slight deviations from axisymmetry can lead to particles that undergo chaotic rotational and translational motions in a simple shear flow, leading to a self-dispersive behavior in the absence of Brownian motion or particle-particle interactions. Non-fore-aft symmetric rings that flow-align and exhibit cross-stream drift will be fabricated using photolithography. To produce fore-aft symmetric rings that align but translate with the fluid motion, the investigators will explore a hybrid synthesis based on producing toroidal particles in a microfluidic drop process and subsequently embedding the particles in a film and stretching to create the sharp outer edge of the ring. The three-dimensional translational and rotational motion of the particles will be observed in a counter-rotating Couette device. The rheology of dilute and semi-dilute suspensions will be measured to reveal the decrease in viscosity due to flow alignment.
