We present an optically driven colloidal microscopic rheometer based on the recent work of Williams et al. The interplay between magnetic and optical forces allows us to experimentally build a two dimensional Taylor-Couette cell and to explore the rheological properties of confined colloidal suspensions at a microscopic scale. Despite the discrete nature of the system, we observe local instabilities in the response of the layers’ flow to the applied shear, which is a characteristic of shear banding in larger systems. Besides the rheological phenomena, these type of experiments can be useful to develop the understanding of non-equilibrium many body systems.
Artificial spin-ice systems have been used to date as microscopic models of frustration induced by lattice topology, as they allow for the direct visualization of spin arrangements and textures. However, the engineering of frustrated ice states in which individual spins can be manipulated in situ and the real-time observation of their collective dynamics remain both challenging tasks. Recently, an analogue system has been proposed theoretically, where an optical landscape confined colloidal particles that interacted electrostatically. Here we realize experimentally another version of a colloidal artificial ice system using interacting magnetically polarizable particles confined to lattices of bistable gravitational traps.
We show quantitatively that ice-selection rules emerge in this frustrated soft matter system by tuning the strength of the pair-interactions between the microscopic units. By using optical tweezers, we can control particle positioning and dipolar coupling, we introduce monopole-like defects and strings and use loops with defined chirality as an elementary unit to store binary information.
The ellipsoidal coordinate system has the interesting property that every other orthogonal coordinate system in which the three-dimensional Helmholtz equation is separable, is a special case of it. In this work, we explore the solutions to the wave equation in ellipsoidal coordinates in order to visualize the behavior of optical fields with ellipsoidal geometry. We show several parity properties which allow us to create fundamental modes of vibration with different symmetries around the (x; y), (x; z) and (y; z) planes. We discuss the resonant modes of an ellipsoidal cavity and the traveling waves with ellipsoidal geometry. We propose a method to calculate the second linearly independent solution to the ellipsoidal wave equation