Julia Yeomans OBE FRS

Abstract:

Meso-scale turbulence is an innate phenomenon, distinct from inertial turbulence, that spontaneously occurs at low-Reynolds number in fluidized biological systems. This spatio-temporal disordered flow radically changes nutrient and molecular transport in living fluids and can strongly affect the collective behaviour in prominent biological processes, including biofilm formation, morphogenesis and cancer invasion. Despite its crucial role in such physiological processes, understanding meso-scale turbulence and any relation to classical inertial turbulence remains obscure. Here, we show how the motion of active matter along a micro-channel transitions to mesoscale turbulence through the evolution of locally disordered patches (active puffs) from an ordered vortex-lattice flow state. We demonstrate that the stationary critical exponents of this transition to meso-scale turbulence in a channel coincide with the directed percolation universality class. This finding bridges our understanding of the onset of low-Reynolds number meso-scale turbulence and traditional scaleinvariant turbulence, therefore generalizing theories on the onset of turbulence in confinement to the distinct classes of incoherent flows observed in biological fluids.

Abstract:

Advances in controlling and exploiting the wetting and adsorption properties of complex fluids, such as liquid crystals, ionic liquids, colloids and active matter, have been fostered by impressive technical achievements allowing the fabrication of tailored surfaces with a well-controlled distribution of micro- or nano-scale features. Such patterned substrates may be used to control the properties of adsorbed fluids in ways relevant to applications including microfluidic devices, surfaces with switchable wettability, new generation liquid crystal displays, or supercapacitors for efficient energy storage. In this special issue we collect together experimental, theoretical and computational papers that showcase recent contributions to understanding complex fluids at structured surfaces. These underline the diversity of phenomena encountered when complex fluids interact with complex surfaces.

Abstract:

Active materials, bacteria, molecular motors, and self-propelled colloids, continuously transform chemical energy from the environment to mechanical work. Dense active matter, from layers of cells to flocks of birds, self-assembles into intricate patterns. Nature’s engines are complex and efficient, and we would like to exploit her ideas to fabricate nano-machines.

Abstract:

The spontaneous emergence of collective flows is a generic property of active fluids and often leads to chaotic flow patterns characterised by swirls, jets, and topological disclinations in their orientation field. However, the ability to achieve structured flows and ordered disclinations is of particular importance in the design and control of active systems. By confining an active nematic fluid within a channel, we find a regular motion of disclinations, in conjunction with a well defined and dynamic vortex lattice. As pairs of moving disclinations travel through the channel, they continually exchange partners producing a dynamic ordered state, reminiscent of Ceilidh dancing. We anticipate that this biomimetic ability to self-assemble organised topological disclinations and dynamically structured flow fields in engineered geometries will pave the road towards establishing new active topological microfluidic devices.