Confinement of surface spinners in liquid metamaterials

Abstract

We show that rotating particles at the liquid–gas interface can be efficiently manipulated using the surface-wave analogue of optical lattices. Two orthogonal standing waves generate surface flows of counter-rotating half-wavelength unit cells, the liquid interface metamaterial, whose geometry is controlled by the wave phase shift. Here we demonstrate that by placing active magnetic spinners inside such metamaterials, one makes a powerful tool which allows manipulation and self-assembly of spinners, turning them into vehicles capable of transporting matter and information between autonomous metamaterial unit cells. We discuss forces acting on a spinner carried by a nonuniform flow and show how the forces confine spinners to orbit inside the same-sign vortex cells of the wave-driven flow. Reversing the spin, we move the spinner into an adjacent cell. By changing the spinning frequency or the wave amplitude, one can precisely control the spinner orbit. Multiple spinners within a unit cell self-organize into stable patterns, e.g., triangles or squares, orbiting around the center of the cell. Spinners having different frequencies can also be confined, such that the higher-frequency spinner occupies the inner orbit and the lower-frequency one circles on the outer orbit, while the orbital motions of both spinners are synchronized.

Roadmap on structured waves

“…The existence of the vertical and horizontal spin in 3D waves is also important for the motion of larger inertial particles possessing internal spin. The motion of such particles is governed by the interaction between the wave spin and particle spin, and it opens the opportunity to manipulate and sort spinning particles using structured surface waves [299]. The existence of the vertical spin in some wave configurations offers a new conceptual base for the development of the surface wave spintronics [300], where the spin of passive particles can be controlled by imposed surface waves [66].…”

Collective Vortical Motion and Vorticity Reversals of Self-Propelled Particles on Circularly Patterned Substrates

“…Many studies have shown that clustering and collective motion of self-propelled particles (SPPs) are influenced by various physical factors, including the packing fraction of the SPPs, nature of the coupling between neighboring SPPs, and the type of motion of a single SPP [20,21]. Other physical factors include environmental constraints [3] such as anisotropy of the embedding fluid [22,23], geometric confinement [24][25][26][27][28][29][30][31][32][33][34][35][36] and obstacles [37,38]. An interesting effect of circular confinement, for example, is an induced vortical motion of the SPPs * Physical Review E (in press)…”

Rolling spinners on the water surface

“…At microscale, i.e., at the Reynolds numbers of Re < 1, colloidal building blocks rotated using external magnetic field can be directed along user-defined paths, thus acting as colloidal microwheels ( 24 ). Recently, it was shown that water surface waves generating circular flows and polarizations ( 25 ) can efficiently interact with subwavelength spinners, which allows efficient manipulation of particles on the water surface ( 26 ). Therefore, understanding the dynamics of spinning particles in fluids constitutes a fundamental and applied problem important for physics and engineering.…”