Browsing by Author "Dietrich, Siegfried"

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Open Access
Correlations and forces in sheared fluids with or without quenching
(2019) Rohwer, Christian M.; Maciołek, Anna; Dietrich, Siegfried; Krüger, Matthias
Spatial correlations play an important role in characterizing material properties related to non-local effects. Inter alia, they can give rise to fluctuation-induced forces. Equilibrium correlations in fluids provide an extensively studied paradigmatic case, in which their range is typically bounded by the correlation length. Out of equilibrium, conservation laws have been found to extend correlations beyond this length, leading, instead, to algebraic decays. In this context, here we present a systematic study of the correlations and forces in fluids driven out of equilibrium simultaneously by quenching and shearing, both for non-conserved as well as for conserved Langevin-type dynamics. We identify which aspects of the correlations are due to shear, due to quenching, and due to simultaneously applying both, and how these properties depend on the correlation length of the system and its compressibility. Both shearing and quenching lead to long-ranged correlations, which, however, differ in their nature as well as in their prefactors, and which are mixed up by applying both perturbations. These correlations are employed to compute non-equilibrium fluctuation-induced forces in the presence of shear, with or without quenching, thereby generalizing the framework set out by Dean and Gopinathan. These forces can be stronger or weaker compared to their counterparts in unsheared systems. In general, they do not point along the axis connecting the centers of the small inclusions considered to be embedded in the fluctuating medium. Since quenches or shearing appear to be realizable in a variety of systems with conserved particle number, including active matter, we expect these findings to be relevant for experimental investigations.
Open Access
Current-mediated synchronization of a pair of beating non-identical flagella
(2019) Dotsenko, Vladimir S.; Maciolek, Anna; Oshanin, Gleb; Vasilyev, Oleg; Dietrich, Siegfried
The basic phenomenology of experimentally observed synchronization (i.e. a stochastic phase locking) of identical, beating flagella of a biflagellate alga is known to be captured well by a minimal model describing the dynamics of coupled, limit-cycle, noisy oscillators (known as the noisy Kuramoto model). As demonstrated experimentally, the amplitudes of the noise terms therein, which stem from fluctuations of the rotary motors, depend on the flagella length. Here we address the conceptually important question which kind of synchrony occurs if the two flagella have different lengths such that the noises acting on each of them have different amplitudes. On the basis of a minimal model, too, we show that a different kind of synchrony emerges, and here it is mediated by a current carrying, steady-state; it manifests itself via correlated ‘drifts’ of phases. We quantify such a synchronization mechanism in terms of appropriate order parameters Q and Qs-for an ensemble of trajectories and for a single realization of noises of duration s, respectively. Via numerical simulations we show that both approaches become identical for long observation times S. This reveals an ergodic behavior and implies that a single-realization order parameter Qs is suitable for experimental analysis for which ensemble averaging is not always possible.
Open Access
Nanoalignment by critical Casimir torques
(2024) Wang, Gan; Nowakowski, Piotr; Farahmand Bafi, Nima; Midtvedt, Benjamin; Schmidt, Falko; Callegari, Agnese; Verre, Ruggero; Käll, Mikael; Dietrich, Siegfried; Kondrat, Svyatoslav; Volpe, Giovanni
The manipulation of microscopic objects requires precise and controllable forces and torques. Recent advances have led to the use of critical Casimir forces as a powerful tool, which can be finely tuned through the temperature of the environment and the chemical properties of the involved objects. For example, these forces have been used to self-organize ensembles of particles and to counteract stiction caused by Casimir-Liftshitz forces. However, until now, the potential of critical Casimir torques has been largely unexplored. Here, we demonstrate that critical Casimir torques can efficiently control the alignment of microscopic objects on nanopatterned substrates. We show experimentally and corroborate with theoretical calculations and Monte Carlo simulations that circular patterns on a substrate can stabilize the position and orientation of microscopic disks. By making the patterns elliptical, such microdisks can be subject to a torque which flips them upright while simultaneously allowing for more accurate control of the microdisk position. More complex patterns can selectively trap 2D-chiral particles and generate particle motion similar to non-equilibrium Brownian ratchets. These findings provide new opportunities for nanotechnological applications requiring precise positioning and orientation of microscopic objects.
Open Access
Non-Markovian description of the Hedvall effect
(1987) Seifert, Udo; Dietrich, Siegfried
Activated processes at surfaces - like desorption or oxidation - exhibit thermal anomalies at phase transitions or the underlying substrate. Inter alia, such singularities in the case of a continuous transition are caused by the critical slowing down in the substrate, which leads to pronounced memory effects in the viscosity coefficient. Therefore, we apply a non-Markovian analog of Kramers' classical rate theory. As a result, the anomalies can be expressed in terms of critical exponents associated with the critical surface behaviour.
Open Access
Shape-dependent guidance of active Janus particles by chemically patterned surfaces
(2018) Uspal, William E.; Popescu, Mihail Nicolae; Tasinkevych, Mykola; Dietrich, Siegfried
Self-phoretic chemically active Janus particles move by inducing-via non-equilibrium chemical reactions occurring on their surfaces-changes in the chemical composition of the solution in which they are immersed. This process leads to gradients in chemical composition along the surface of the particle, as well as along any nearby boundaries, including solid walls. Chemical gradients along a wall can give rise to chemi-osmosis, i.e., the gradients drive surface flows which, in turn, drive flow in the volume of the solution. This bulk flow couples back to the particle, and thus contributes to its self-motility. Since chemi-osmosis strongly depends on the molecular interactions between the diffusing molecular species and the wall, the response flow induced and experienced by a particle encodes information about any chemical patterning of the wall. Here, we extend previous studies on self-phoresis of a sphere near a chemically patterned wall to the case of particles with rod-like, elongated shape. We focus our analysis on the new phenomenology potentially emerging from the coupling-which is inoperative for a spherical shape-of the elongated particle to the strain rate tensor of the chemi-osmotic flow. Via detailed numerical calculations, we show that the dynamics of a rod-like particle exhibits a novel ‘edge-following’ steady state: the particle translates along the edge of a chemical step at a steady distance from the step and with a steady orientation. Moreover, within a certain range of system parameters, the edge-following state co-exists with a ‘docking’ state (the particle stops at the step, oriented perpendicular to the step edge), i.e., a bistable dynamics occurs. These findings are rationalized as a consequence of the competition between the fluid vorticity and the rate of strain by using analytical theory based on the point-particle approximation which captures quasi-quantitatively the dynamics of the system.