76 research outputs found

    Emergent behavior in active colloids

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    Active colloids are microscopic particles, which self-propel through viscous fluids by converting energy extracted from their environment into directed motion. We first explain how articial microswimmers move forward by generating near-surface flow fields via self-phoresis or the self-induced Marangoni effect. We then discuss generic features of the dynamics of single active colloids in bulk and in confinement, as well as in the presence of gravity, field gradients, and fluid flow. In the third part, we review the emergent collective behavior of active colloidal suspensions focussing on their structural and dynamic properties. After summarizing experimental observations, we give an overview on the progress in modeling collectively moving active colloids. While active Brownian particles are heavily used to study collective dynamics on large scales, more advanced methods are necessary to explore the importance of hydrodynamic and phoretic particle interactions. Finally, the relevant physical approaches to quantify the emergent collective behavior are presented.Comment: 31 pages, 14 figure

    Self-propelled droplet driven by Marangoni flow and its applications

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    We developed a new class of self-propelled droplet, which is made of water/ethanol dispersed in squalane/monoolein. During the propulsion, a spontaneous phase separation of the droplet occurs due to the release of ethanol and the uptake of monoolein. This phase separation can lead to the formation of a Janus droplet consisting of a water-rich phase and an ethanol-rich phase. The droplet moves as a pusher, which is determined by µPIV, before the phase separation and as a neutral squirmer after it. The time before phase separation can be quantified by a model. Additionally the quantitative analysis of the driving mechanisms before and after the phase separation are presented. Depending on salt concentration, added DNA or RNA can be controlled to accumulate either in the water-rich or in the ethanol-rich phase as a 'cargo'. This 'cargo' can be selectively delivered to a target controlled by hydrodynamic interaction and wettability. The same water/ethanol droplet in an ethanol-saturated squalane shows chemotaxic attraction. In this system, the droplet uptakes ethanol from squalane and droplets are attracted to each other supposably driven by this ethanol gradient, which is created by themselves. Large numbers of droplets can form patterns with different shapes, which is controlled by number density and vertical confinement.Wir haben eine neue Klasse von selbst-angetriebenen Tropfen entwickelt, die aus Wasser / Ethanol bestehen und in Squalan / Monoolein dispergiert sind. Während der Bewegung kommt es zu einer spontanen Phasentrennung des Tropfens aufgrund der Abgabe von Ethanol und der Aufnahme von Monoolein. Diese Phasentrennung kann zur Ausbildung eines Janus-Tropfens führen, der aus einer wasserreichen Phase und einer ethanolreichen Phase besteht. Der Tropfen bewegt sich vor der Phasentrennung als 'Pusher', der durch µPIV bestimmt wird, und danach als neutraler 'Squirmer'. Die Zeit vor der Phasentrennung kann durch ein Modell quantifiziert. Zusätzlich wird die quantitative Analyse der Antriebsmechanismen vor und nach der Phasentrennung vorgestellt. Abhängig von der Salzkonzentration kann die zugesetzte DNA oder RNA so gesteuert werden, dass sie sich entweder in der wasserreichen oder in der ethanolreichen Phase als "Ladung" ansammelt. Diese 'Ladung' kann selektiv an ein Ziel geliefert werden und durch hydrodynamische Wechselwirkung und Benetzbarkeit gesteuert werden. Der gleiche Wasser/Ethanol Tropfen in ethanolgesättigten Squalan zeigt eine chemotaktische Anziehungskraft. In diesem System nehmen die Tropfen Ethanol aus Squalan auf und die Tropfen werden vermutlich durch diesen Ethanolgradienten, der von ihnen selbst erzeugt wird, voneinander angezogen. Die Ansammlung vieler Tropfen können Muster bilden. Das Muster wird von der Tropfendichte und der vertikalen Ausdehnung kontrolliert

    Active Emulsions: Physicochemical Hydrodynamics and Collective Behavior

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    Active matter is a collection of constituent elements that constantly consume energy, convert it to mechanical work, and interact with their counterparts. These materials operate out of equilibrium and exhibit fascinating collective dynamics such as spontaneous pattern formation. Self-organization of bio-polymers within a cell, collective migration of bacteria in search of nutrition, and the bird flocks are paragons of active living matter and the primary source of our knowledge on it. To understand the overarching physical principles of active matter, it is desirable to build artificial systems that are capable of imitating living active matter while ruling out the biological complexities. The goal of this thesis is to study active micro-droplets as a paradigm for biomimetic artificial active particles, using fundamental principles of fluid dynamics and statistical physics. The Marangoni-driven motility in these droplets is reminiscent of the locomotion of some protozoal organisms, known as squirmers. The main scientific objectives of this research are to (i) investigate the potential biomimetic features of active droplets including compartmentalization, adaptability (e.g. multi-gait motility), and information processing (signaling and sensing) and (ii) study the implications of those features in the collective dynamics of active emulsions governed by hydrodynamic and autochemotactic interactions. These objectives are addressed experimentally using microfluidics and microscopy, integrated with quantitative image analysis. The quantitative experimental results are then compared with the predictions from theory or simulations. The findings of this thesis are presented in five chapters. First, we address the challenge of compartmentalizing active droplets. We use microfluidics to generate liquid shells (double emulsions). We propose and successfully prove the use of a nematic liquid crystal oil to stabilize the liquid shells, which are otherwise susceptible to break up during motility. We investigate the propulsion dynamics and use that insight to put forward routes to control shell motion via topology, chemical signaling, and topography. In the second results chapter, we establish the bimodal dynamics of chaotic motility in active droplets; a regime that emerges as a response to the increase of viscosity in the swimming medium. To establish the physical mechanism of this dynamical transition, we developed a novel technique to simultaneously visualize the hydrodynamic and chemical fields around the droplet. The results are rationalized by quantitative comparison to established advection-diffusion models. We further observe that the droplets undergo self-avoiding random walks as a result of interaction with the self-generated products of their activity, secreted in the environment. The third results chapter presents a review of the dynamics of chemotactic droplets in complex environments, highlighting the effects of self-generated chemical interactions on the droplet dynamics. In the fourth results chapter, we investigate how active droplets sense and react to the chemical gradients generated by their counterparts--- a behavior known as autochemotaxis. Then, we study the collective dynamics governed by these autochemotactic interactions, in two and three dimensions. For the first time, we report the observation of ‘history caging’, where swimmers are temporarily trapped in an evolving network of repulsive chemical trails. The caging results in a plateau in the mean squared displacement profiles as observed for dense colloidal systems near the glass transition. In the last results chapter, we investigate the collective dynamics in active emulsions, governed by hydrodynamic interactions. We report the emergence of spontaneously rotating clusters. We show that the rotational dynamics originates from a novel symmetry breaking mechanism for single isotropic droplets. By extending our understanding to the collective scale, we show how the stability and dynamics of the clusters can be controlled by droplet activity and cluster size. The experimental advancements and the findings presented in this thesis lay the groundwork for future investigations of emergent dynamics in active emulsions as a model system for active matter. In the outlook section, we present some of the new questions that have developed in the course of this research work and discuss a perspective on the future directions of the research on active droplets.2022-01-1

    Simulating squirmers with multiparticle collision dynamics

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    Multiparticle collision dynamics is a modern coarse-grained simulation technique to treat the hydrodynamics of Newtonian fluids by solving the Navier-Stokes equations. Naturally, it also includes thermal noise. Initially it has been applied extensively to spherical colloids or bead-spring polymers immersed in a fluid. Here, we review and discuss the use of multiparticle collision dynamics for studying the motion of spherical model microswimmers called squirmers moving in viscous fluids.Comment: 11 pages, 6 figures, open access articl

    キラルな液晶の非平衡ダイナミクス

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    学位の種別: 課程博士審査委員会委員 : (主査)東京大学教授 山室 修, 東京大学教授 宮下 精二, 産業技術総合研究所主任研究員 福田 順一, 東京大学准教授 野口 博司, 千葉大学准教授 北畑 裕之University of Tokyo(東京大学

    Tank-treading as a means of propulsion in viscous shear flows

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    The use of tank-treading as a means of propulsion for microswimmers in viscous shear flows is taken into exam. We discuss the possibility that a vesicle be able to control the drift in an external shear flow, by varying locally the bending rigidity of its own membrane. By analytical calculation in the quasi-spherical limit, the stationary shape and the orientation of the tank-treading vesicle in the external flow, are determined, working to lowest order in the membrane inhomogeneity. The membrane inhomogeneity acts in the shape evolution equation as an additional force term, that can be used to balance the effect of the hydrodynamic stresses, thus allowing the vesicle to assume shapes and orientations that would otherwise be forbidden. The vesicle shapes and orientations required for migration transverse to the flow, together with the bending rigidity profiles that would lead to such shapes and orientations, are determined. A simple model is presented, in which a vesicle is able to migrate up or down the gradient of a concentration field, by stiffening or softening of its membrane, in response to the variations in the concentration level experienced during tank-treading.Comment: 21 pages, 4 figure

    Arrested on heating: controlling the motility of active droplets by temperature

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    One of the challenges in tailoring the dynamics of active, self-propelling agents lies in arresting and releasing these agents at will. Here, we present an experimental system of active droplets with thermally controllable and reversible states of motion, from unsteady over meandering to persistent to arrested motion. These states depend on the P\'eclet number of the chemical reaction driving the motion, which we can tune by using a temperature sensitive mixture of surfactants as a fuel medium. We quantify the droplet dynamics by analysing flow and chemical fields for the individual states, comparing them to canonical models for autophoretic particles. In the context of these models, we are able to observe in situ the fundamental first transition between the isotropic, immotile base state and self-propelled motility
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