Person: Lozano, Celia
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Role of cohesion in the flow of active particles through bottlenecks
We experimentally and numerically study the flow of programmable active particles (APs) with tunable cohesion strength through geometric constrictions. Similar to purely repulsive granular systems, we observe an exponential distribution of burst sizes and power-law-distributed clogging durations. Upon increasing cohesion between APs, we find a rather abrupt transition from an arch-dominated clogging regime to a cohesion-dominated regime where droplets form at the aperture of the bottleneck. In the arch-dominated regime the flow-rate only weakly depends on the cohesion strength. This suggests that cohesion must not necessarily decrease the group’s efficiency passing through geometric constrictions or pores. Such behavior is explained by “slippery” particle bonds which avoids the formation of a rigid particle network and thus prevents clogging. Overall, our results confirm the general applicability of the statistical framework of intermittent flow through bottlenecks developed for granular materials also in case of active microswimmers whose behavior is more complex than that of Brownian particles but which mimic the behavior of living systems.
Autonomously Probing Viscoelasticity in Disordered Suspensions
Recent experiments show a strong rotational diffusion enhancement for self-propelled microrheological probes in colloidal glasses. Here, we provide microscopic understanding using simulations with a frictional probe-medium coupling that converts active translation into rotation. Diffusive enhancement emerges from the medium’s disordered structure and peaks at a second-order transition in the number of contacts. Our results reproduce the salient features of the colloidal glass experiment and support an effective description that is applicable to a broader class of viscoelastic suspensions.
Colloidal Brazil nut effect in microswimmer mixtures induced by motility contrast
We numerically and experimentally study the segregation dynamics in a binary mixture of microswimmers which move on a two-dimensional substrate in a static periodic triangular-like light intensity field. The motility of the active particles is proportional to the imposed light intensity, and they possess a motility contrast, i.e., the prefactor depends on the species. In addition, the active particles also experience a torque aligning their motion towards the direction of the negative intensity gradient. We find a segregation of active particles near the intensity minima where typically one species is localized close to the minimum and the other one is centered around in an outer shell. For a very strong aligning torque, there is an exact mapping onto an equilibrium system in an effective external potential that is minimal at the intensity minima. This external potential is similar to (height-dependent) gravity such that one can define effective “heaviness” of the self-propelled particles. In analogy to shaken granular matter in gravity, we define a “colloidal Brazil nut effect” if the heavier particles are floating on top of the lighter ones. Using extensive Brownian dynamics simulations, we identify system parameters for the active colloidal Brazil nut effect to occur and explain it based on a generalized Archimedes’ principle within the effective equilibrium model: heavy particles are levitated in a dense fluid of lighter particles if their effective mass density is lower than that of the surrounding fluid. We also perform real-space experiments on light-activated self-propelled colloidal mixtures which confirm the theoretical predictions.
Run-and-Tumble-Like Motion of Active Colloids in Viscoelastic Media
Run-and-tumble motion is a prominent locomotion strategy employed by many living microorganisms. It is characterized by straight swimming intervals (runs), which are interrupted by sudden reorientation events (tumbles). In contrast, directional changes of synthetic microswimmers (active particles) are caused by rotational diffusion, which is superimposed with their translational motion and thus leads to rather continuous and slow particle reorientations. Here we demonstrate that active particles can also perform a swimming motion where translational and orientational changes are disentangled, similar to run-and-tumble. In our system, such motion is realized by a viscoelastic solvent and a periodic modulation of the self-propulsion velocity. Experimentally, this is achieved using light-activated Janus colloids, which are illuminated by a time-dependent laser field. We observe a strong enhancement of the effective translational and rotational motion when the modulation time is comparable to the relaxation time of the viscoelastic fluid. Our findings are explained by the relaxation of the elastic stress, which builds up during the self-propulsion, and is suddenly released when the activity is turned off. In addition to a better understanding of active motion in viscoelastic surroundings, our results may suggest novel steering strategies for synthetic microswimmers in complex environments.
Realization of a motility-trap for active particles
Trapping of atomic and mesoscopic particles with optical fields is a practical technique employed in many research disciplines. Developing similar trapping methods for self-propelled, i.e. active, particles is, however, challenging due to the typical anisotropic material composition of Janus-type active particles. This renders their trapping with magneto-optical fields to be difficult. Here we present the realization of a motility-trap for active particles, which only exploits their self-propulsion properties. By combining experiments, numerical simulations, and theory, we show that, under appropriate conditions, a force-free rotation of the self-propulsion direction towards the trap's center can be achieved, which results in an exponential localization of active particles. Because this trapping mechanism can be applied to any propulsion scheme, we expect such motility-tweezers to be relevant for fundamental studies of self-driven objects as well as for their applications as autonomous microrobots.
Active particles sense micromechanical properties of glasses
Understanding the mechanical properties of glasses is a great scientific challenge. A powerful technique to study the material response on a microscopic scale is microrheology, in which one analyses the translational dynamics of an externally driven probe particle. Here we show that the translational and rotational dynamics of a self-propelled probe particle with an unconstrained orientational motion can be used to gather information about the mechanical properties of a colloidal glassy system. We find that its rotational diffusion coefficient continuously increases towards the glass transition and drops down in the glassy state. Such unexpected behaviour demonstrates a strong coupling mechanism between the orientation of the active probe particle and the glassy structure, which can be well described by a simple rheological model. Our results suggest that active probe particles may be useful for the micromechanical characterization of complex materials.
Diffusing wave paradox of phototactic particles in traveling light pulses
Cells navigate through complex surroundings by following cues from their environment. A prominent example is Dictyostelium, which is directed by chemotaxis towards regions with higher concentrations. In the presence of traveling chemical waves, however, amoebae migrate counter to the running wave. Such behavior, referred to as diffusing wave paradox, suggests the existence of adaptation and directional memory. Here we experimentally investigate the response of phototactic self-propelled microparticles to traveling light-pulses. Despite their entirely memory-less (i.e., strictly local) response to the environment, we observe the same phenomenological behavior, i.e., particle motion counter to the pulse direction. Our findings are supported by a minimal model which considers active particle reorientations within local light gradients. The complex and robust behavior of synthetic active particles to spatially and temporally varying stimuli enables new strategies for achieving collective behavior and can be used for the design of micro-robotic systems with limited signal-processing capabilities.
Direct-imaging of light-driven colloidal Janus particles in weightlessness
We describe a highly integrated automated experiment module that allows us to investigate the active Brownian motion of light-driven colloidal Janus-particle suspensions. The module RAMSES (RAndom Motion of SElf-propelled particles in Space) is designed for the sounding rocket platform MAPHEUS (MAterialPHysikalische Experimente Unter Schwerelosigkeit). It allows us to perform experiments under weightlessness conditions in order to avoid sedimentation of the Janus particles and thus to study the spatially three-dimensional dynamics in the suspension. The module implements a newly developed strong homogeneous light source to excite self-propulsion in the Janus particles. The light source is realized through an array of high-power light-emitting diodes and replaces the conventional laser source, thus reducing heat dissipation and spatial extension of the experiment setup. The rocket module contains ten independent sample cells in order to ease the systematic study of the effect of control parameters such as light intensity or particle concentration and size in a single sounding-rocket flight. For each sample cell, transmitted light intensities are stored for postflight analysis in terms of differential dynamical microscopy.
Propagating density spikes in light-powered motility-ratchets
Combining experiments and computer simulations, we use a spatially periodic and flashing light-field to direct the motion of phototactic active colloids. Here, the colloids self-organize into a density spike pattern, which resembles a shock wave and propagates over long distances, almost without dispersing. The underlying mechanism involves a synchronization of the colloids with the light-field, so that particles see the same intensity gradient each time the light-pattern is switched on, but no gradient in between (for example). This creates pulsating transport whose strength and direction can be controlled via the flashing protocol and the self-propulsion speed of the colloids. Our results might be useful for drug delivery applications and can be used to segregate active colloids by their speed.
Slow relaxation dynamics of clogs in a vibrated granular silo
We experimentally explore the vibration-induced unclogging of arches halting the flow in a two-dimensional silo. The endurance of arches is determined by carrying out a survival analysis of their breaking times. By analyzing the dynamics of two morphological variables, we demonstrate that arches evolve toward less regular structures and it seems that there may exist a certain degree of irregularity that the arch reaches before collapsing. Moreover, we put forward that σ (the standard deviation of all angles between consecutive beads) describes faithfully the morphological evolution of the arch. Focusing on long-lasting arches, we study σ calculating its two-time autocorrelation function and its mean-squared displacement. In particular, the apparent logarithmic increase of the correlation and the decrease of the mean-squared displacement of σ when the waiting time is increased reveal a slowing down of the dynamics. This behavior is a clear hallmark of aging phenomena and confirms the lack of ergodicity in the unclogging dynamics. Our findings provide new insights on how an arch tends to destabilize and how the probability that it breaks with a long sustained vibration decreases with time.