Hua Xia

Spin is a fundamental yet nontrivial intrinsic angular momentum property of quantum particles or fields, which appears within relativistic field theory. The spin density in wave fields is described by the theoretical Belinfante-Rosenfeld construction based on the difference between the canonical and kinetic momentum densities. These quantities are usually considered as abstract and non-observable per se. Here, we demonstrate, both theoretically and experimentally, that the Belinfante-Rosenfeld construction naturally arises in gravity (water surface) waves. There, the canonical momentum is associated with the generalized Stokes drift phenomenon, while the spin is generated by subwavelength circular motion of water particles. Thus, we directly observe these fundamental field theory properties as microscopic mechanical properties of a classical wave system. Our findings shed light onto the nature of spin and momentum in wave fields, demonstrate the universality of relativistic field theory concepts, and offer a new platform for their studies.

Bacterial cellulose is an important class of biomaterials which can be grown in well-controlled laboratory and industrial conditions. The cellulose structure is affected by several biological, chemical and environmental factors, including hydrodynamic flows in bacterial suspensions. In this work, we explore the possibility of using well controlled turbulent flow to control the bacterial cellulose production. Turbulent flows leading to random motion of fluid elements may affect the structure of the extracellular polymeric matrix produced by bacteria. Here we show that two-dimensional turbulence at the air-liquid interface generates chaotic rotation at a well-defined scale and random persistent stretching of the fluid elements. The results offer new approaches to engineering of the bacterial cellulose structure by controlling turbulence parameters.

Fast-spinning discs on a liquid surface propagate along arbitrarily shaped boundaries, making a new type of surface vehicles.

We characterize a process of energy extraction via rectification of strongly turbulent flow by using tools of stochastic thermodynamics. We study the dynamics of an asymmetric autonomous rotor that shows biased direction of rotation when placed in a stream. We give experimental evidence that a fluctuation theorem can be used to describe the work injected in the rotor via its coupling with the turbulent flow structure. This approach allows to measure the mean power extracted from the chaotic fluid motion over a broad range of turbulent kinetic energy. A nontrivial dependence of the rotor power on flow kinetic energy is identified. This observation is described by a model taking into account the dissipation of the rotor energy and the temporal memory of coherent structures present in the turbulent flow.

Wave-controlled transport in fluid provides a novel method to shape and promote biofilm formation for bioengineering applications.

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.

The formation of bacterial biofilms on solid surfaces within a fluid starts when bacteria attach to the substrate. Understanding environmental factors affecting the attachment and the early stages of the biofilm development as well as the development of active methods of biofilm control are crucial for many applications. Here we show that biofilm formation is strongly affected by the hydrodynamics of flows generated by surface waves in layers of bacterial suspensions. Deterministic wave patterns promote the growth of patterned biofilms while wave-driven turbulent motion destroys the patterns. The location of the attached bacteria on a solid substrate differs from the settlement location of inactive bacteria and of the passive micro-particles: strong biofilms form under the wave antinodes while passive particles and inactive bacteria settle under nodal points. By controlling the wave lengths and horizontal mobility of the wave patterns, one can either shape the biofilm formation and enhance the biofilm growth, or discourage the formation of biofilm patterns. The results suggest that the deterministic wave-driven transport, rather than hydrodynamic forces, determine the preferred location for the bacterial attachment.

We report on the transport properties and orientational dynamics of ellipsoidal objects advected by laboratory two-dimensional turbulence. It is found that ellipsoids of different sizes have preferential direction of transport, either along their major axes or minor axes. The two components of the ellipsoid diffusion coefficient depend on the ratio of the length of the ellipsoids along major axes aa to the turbulence forcing scale L_f. Large ellipsoids (aa > L_f) diffuse faster in the direction parallel to their major axes. In contrast, small ellipsoids diffuse faster in the direction transverse to their major axes. We study this transition vs the ratio aa/L_f and relate it to the coupling between translational and rotational motion of anisotropic objects. The features of the turbulent transport of ellipsoids can be understood by considering the interaction of these anisotropic objects with the underlying structure of two dimensional turbulent flows made of meandering coherent bundles.