We experimentally investigate the non-equilibrium steady-state distribution of the work done by an external force on a mesoscopic system with many coupled degrees of freedom: a colloidal crystal mechanically driven across a commensurate periodic light field. Since this system mimics the spatiotemporal dynamics of a crystalline surface moving on a corrugated substrate, our results show general properties of the work distribution for atomically flat surfaces undergoing friction. We address the role of several parameters which can influence the shape of the work distribution, e.g. the number of particles used to locally probe the properties of the system and the time interval to measure the work. We find that, when tuning the control parameters to induce particle depinning from the substrate, there is an abrupt change of the shape of the work distribution. While in the completely static and sliding friction regimes the work distribution is Gaussian, non-Gaussian tails show up due to the spatiotemporal heterogeneity of the particle dynamics during the transition between these two regimes.
We experimentally study the motion of a colloidal monolayer which is driven across a commensurate substrate potential whose amplitude is periodically modulated in time. In addition to a significant reduction of the static friction force compared to an unmodulated substrate, we observe a Shapiro step structure in the force dependence of the mean particle velocity which is explained by the dynamical mode locking between the particle motion and the substrate modulation. In this regime, the entire crystal moves in a stick-slip fashion similar to what is observed when a single point contact is driven across a periodic surface. Contrary to numerical simulations, where typically a large number of Shapiro steps is found, only a single step is observed in our experiments. This is explained by the formation of kinks which weaken the synchronization between adjacent particles.
The depletion interaction between a probe sphere and a flat wall induced by fd-virus is investigated by means of total internal reflection microscopy (TIRM). The viruses serve as a model system for mono-disperse, rod-like colloids. We find that the experimental potentials are well described by the first-order density approximation up to an fd-content of several overlap concentrations. This is in accordance with higher order density theory as confirmed by numerical calculations. Since the first order analytical description still holds for all measurements, this exemplifies that higher order terms of the theory are unimportant for our system.Comparing the potentials induced by wild-type fd to those induced by a more rigid fd variant, it can be shown that the influence of the virus stiffness is beyond our experimental resolution and plays only a negligible role for the measured depletion potentials.
Dynamical properties of colloidal clusters composed of paramagnetic beads are presented. The clusters were trapped either in a parabolic trough or in a hard-wall confinement. In order to access the dynamics of the ensembles, the instantaneous normal mode (INM) approach is utilized, which uses cluster configurations as an input. The peaks in the mode spectra weaken when the system size is increased and when the coupling strength is lowered. The short-time diffusive properties of the clusters are deduced using the INM technique. It is found that angular diffusion is always larger than radial diffusion regardless of the shape of the external trap. Further, short-time diffusion seems to be almost independent of the coupling strength in the solid regime, but decreases with increasing packing fraction and size of the ensembles. In general, it is found that diffusion is larger for parabolically confined than for hard-wall trapped clusters.
We investigate depletion interactions near a wall caused by polydisperse silica-coated gibbsite platelets, using total internal reflection fluorescence microscopy (TIRF) to characterize the sphere-wall interaction potential. As no theoretical model for polydisperse platelets exists, we extend a model for monodisperse depletant cylinders by assuming negligible thickness and averaging over the disc size distribution, finding nearly perfect matching with experimental data. The resulting averaged depletion potentials have, as predicted by the extended model, the same depth as monodisperse potentials, but differ in range and shape according to the size distribution. We compare mean particle sizes and standard deviations derived by TIRF with results from TEM measurements.
We performed total internal reflection microscopy (TIRM) experiments to determine the depletion potentials between probe spheres and a flat glass wall, induced by rod-shaped colloids (fd-virus), and we suggest a new approach to study the spatially resolved dynamics of the probe spheres.
Abstract.We investigate the influence of flow fields on the strength of the depletion interaction caused by disc-shaped depletants. At low mass concentration of discs, it is possible to continuously decrease the depth of the depletion potential by increasing the applied shear rate until the depletion force is not perceivable experimentally. Above a threshold in the platelet mass concentration, the depletion potential can no longer be affected by flow in the accessible range of shear rates. While the observed decrease of depletion strength at low depletant concentration may be ascribed to flow alignment of the discs, it is not clear why the influence of flow is vanishing at high concentrations. In order to observe these effects, a modification of the established total internal reflexion microscopy (TIRM) technique is be implemented. We show the suitability of these modifications to measure particle-wall interaction potentials under non-equilibrium conditions for systems where particles are exposed to a shear.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.