Condensed Matter Theory

Abstract:

We propose a theoretical model for a magnetically-actuated artificial cilium in a fluid environment and investigate its dynamical behaviour, using both analytical calculations and numerical simulations. The cilium consists of a spherical soft magnet, a spherical hard magnet, and an elastic spring that connects the two magnetic components. Under a rotating magnetic field, the cilium exhibits a transition from phase-locking at low frequencies to phase-slipping at higher frequencies. We study the dynamics of the magnetic cilium in the vicinity of a wall by incorporating its hydrodynamic influence, and examine the efficiency of the actuated cilium in pumping viscous fluids. This cilium model can be helpful in a variety of applications such as transport and mixing of viscous solutions at small scales and fabricating microswimmers.

Abstract:

Motivated by the implications of the complex and dynamic modular geometry of an enzyme on its motion, we investigate the effect of combining long-range internal and external hydrodynamic interactions due to thermal fluctuations with short-range surface interactions. An asymmetric dumbbell consisting of two unequal subunits, in a nonuniform suspension of a solute with which it interacts via hydrodynamic interactions as well as non-contact surface interactions, is shown to have two alignment mechanisms due to the two types of interactions. In addition to alignment, the chemical gradient results in a drift velocity that is modified by hydrodynamic interactions between the constituents of the enzyme.

Abstract:

Eigenstates of fully many-body localized (FMBL) systems can be organized into spin algebras based on quasilocal operators called l-bits. These spin algebras define quasilocal l-bit measurement (τzi) and l-bit flip (τxi) operators. For a disordered Heisenberg spin chain in the MBL regime we approximate l-bit flip operators by finding them exactly on small windows of systems and extending them onto the whole system by exploiting their quasilocal nature. We subsequently use these operators to represent approximate eigenstates. We then describe a method to calculate products of local observables on these eigenstates for systems of size L in O(L2) time. This algorithm is used to compute the error of the approximate eigenstates.

Abstract:

© 2019, The Author(s), under exclusive licence to Springer Nature Limited. The locomotion of swimming bacteria in simple Newtonian fluids can successfully be described within the framework of low-Reynolds-number hydrodynamics 1 . The presence of polymers in biofluids generally increases the viscosity, which is expected to lead to slower swimming for a constant bacterial motor torque. Surprisingly, however, experiments have shown that bacterial speeds can increase in polymeric fluids 2–5 . Whereas, for example, artificial helical microswimmers in shear-thinning fluids 6 or swimming Caenorhabditis elegans worms in wet granular media 7,8 increase their speeds substantially, swimming Escherichia coli bacteria in polymeric fluids show just a small increase in speed at low polymer concentrations, followed by a decrease at higher concentrations 2,4 . The mechanisms behind this behaviour are currently unclear, and therefore we perform extensive coarse-grained simulations of a bacterium swimming in explicitly modelled solutions of macromolecular polymers of different lengths and densities. We observe an increase of up to 60% in swimming speed with polymer density and demonstrate that this is due to a non-uniform distribution of polymers in the vicinity of the bacterium, leading to an apparent slip. However, this in itself cannot predict the large increase in swimming velocity: coupling to the chirality of the bacterial flagellum is also necessary.