|dc.description.abstract||Active ﬂuids are ubiquitous in nature, spanning both microscopic and macroscopic length scales. They belong to an interesting new class of nonequilibrium systems in physics. In contrast with externally driven materials, active ﬂuids are intrinsically forced out of equilibrium by their constituent active particles, which consume energy from the environment, using it to engage in nonequilibrium activities, such as motility and replication. Two of the most prevalent examples of active particles are bacteria and actomyosin complexes. In both cases, we observe rich collective behaviour giving rise to, respectively, the multicellular organisation of bacterial colonies and the intracellular structure in eukaryotes.
A hydrodynamic description can be used to model the behaviour of active ﬂuids. This approach is based on the fact that the active particles exert stresses on the surrounding ﬂuid as they move through it. Within the hydrodynamic framework, we can study many interesting nonequilibrium phenomena, including hydrodynamic instabilities and spontaneous symmetry breaking. These provide an explanation for macroscopic motility patterns in active systems. In our work, we use a hybrid lattice Boltzmann method to simulate a range of motile states and their chiral characteristics.
We start by studying the dynamics of an active ﬂuid enclosed in a droplet with imposed orientational anchoring at the interface. Our results show that when the anchoring is strong enough, active extensile and contractile stresses lead to spontaneous droplet rotation. In contrast, if the anchoring is weak, the droplet instead translates. The signature of the observed rotating states is a signiﬁcant deformation of the droplet shape, distinguishing it from the rotation of spiral defects, which has been previously reported. Moreover, the sense of rotation is selected via spontaneous symmetry breaking, so that the droplet is equally likely to start rotating clockwise or anticlockwise, acquiring a random chirality. Most biological active particles are also microscopically chiral themselves. Thus, active processes must involve chiral interactions at the microscale. We address this feature by considering a chiral active stress originating from a collection of active torque dipoles. We ﬁnd that the active chiral stresses drive a nonequilibrium transition to a self-assembled cholesteric phase in both active ﬂuids and dry active systems. This spontaneously twisted phase shares many of the characteristics of equilibrium cholesterics, including the formation of layered and ﬁngering patterns, and the existence on non-singular defects. If the activity is suciently high, chiral active stresses are capable of untwisting an equilibrium cholesteric conﬁguration (which is thermodynamically favoured).
Finally, we look at active ﬂuids with both achiral (extensile, contractile) and chiral active stresses. We observe that contractile activity suppresses the spontaneous twist, while extensile activity enhances it. We also show the existence of a pitchsplay instability in these systems, leading to a bend deformation of cholesteric layers.||en