Gravitaxis of asymmetric self-propelled colloidal particles

Many motile microorganisms adjust their swimming motion relative to the gravitational field and thus counteract sedimentation to the ground. This gravitactic behavior is often the result of an inhomogeneous mass distribution which aligns the microorganism similar to a buoy. However, it has been suggested that gravitaxis can also result from a geometric fore-rear asymmetry, typical for many self-propelling organisms. Despite several attempts, no conclusive evidence for such an asymmetry-induced gravitactic motion exists. Here, we study the motion of asymmetric self-propelled colloidal particles which have a homogeneous mass density and a well-defined shape. In experiments and by theoretical modeling we demonstrate that a shape anisotropy alone is sufficient to induce gravitactic motion with either preferential upward or downward swimming. In addition, also trochoid-like trajectories transversal to the direction of gravity are observed.Comment: 9 pages, 5 figures, 1 tabl

Dynamical clustering and phase separation in suspensions of self-propelled colloidal particles

We study experimentally and numerically a (quasi) two dimensional colloidal suspension of self-propelled spherical particles. The particles are carbon-coated Janus particles, which are propelled due to diffusiophoresis in a near-critical water-lutidine mixture. At low densities, we find that the driving stabilizes small clusters. At higher densities, the suspension undergoes a phase separation into large clusters and a dilute gas phase. The same qualitative behavior is observed in simulations of a minimal model for repulsive self-propelled particles lacking any alignment interactions. The observed behavior is rationalized in terms of a dynamical instability due to the self-trapping of self-propelled particles.Comment: 8 pages including supplemental information, to appear in Phys. Rev. Let

Reply to Comment on "Circular Motion of Asymmetric Self-Propelling Particles"

In a Comment [Phys. Rev. Lett. 113, 029801 (2014)] on our Letter on self-propelled asymmetric particles [Phys. Rev. Lett. 110, 198302 (2013); arXiv:1302.5787], Felderhof claims that our theory based on Langevin equations would be conceptually wrong. In this Reply we show that our theory is appropriate, consistent, and physically justified.Comment: 2 page

Active Brownian Motion Tunable by Light

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e. gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this article, we investigate in some more details their swimming mechanism leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture generating a spatial chemical concentration gradient, which is responsible for the particle's self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behaviour of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent

Can the self-propulsion of anisotropic microswimmers be described by using forces and torques?

The self-propulsion of artificial and biological microswimmers (i.e., active colloidal particles) has often been modelled by using a force and a torque entering into the overdamped equations for the Brownian motion of passive particles. This seemingly contradicts the fact that a swimmer is force-free and torque-free, i.e., that the net force and torque on the particle vanish. Using different models for mechanical and diffusiophoretic self-propulsion, we demonstrate here that the equations of motion of microswimmers can be mapped onto those of passive particles with the shape-dependent grand resistance matrix and formally external effective forces and torques. This is consistent with experimental findings on the circular motion of artificial asymmetric microswimmers driven by self-diffusiophoresis. The concept of effective self-propulsion forces and torques significantly facilitates the understanding of the swimming paths, e.g., for a microswimmer under gravity. However, this concept has its limitations when the self-propulsion mechanism of a swimmer is disturbed either by another particle in its close vicinity or by interactions with obstacles, such as a wall.Comment: 19 pages, 2 figure

Active Brownian motion of colloidal particles

Während der letzten Jahrzehnte wurde die aktive Brown'sche Bewegung biologischer Organismen intensiv von Forschern aus verschiedenen Feldern untersucht. Im Gegensatz zu Brown'schen Teilchen ist die Bewegung aktiver Brown'scher Objekte nicht im thermischen Gleichgewicht mit ihrer Umgebung. Sie entziehen ihrem Umfeld Energie und konvertieren diese in eine gerichtete Bewegung. Im Fall biologischer Schwimmer wird dies durch feine, haarartige Strukturen auf ihrer Oberfläche realisiert, welche durch einen integrierten chemischen Motor in Rotation versetzt werden. Aus der hohen Komplexität biologischer Organismen ergeben sich häufig Probleme bei der Erforschung bestimmter Phänomene, da häufig eine Kombination mehrerer physikalischer und physiologischer Mechanismen zu einem Effekt beiträgt. Um ein besseres Verständnis für solche Systeme zu erlangen, begannen Wissenschaftler mit der Herstellung künstlicher Mikroschwimmer mit ähnlichen Eigenschaften. Analog zu biologischen Schwimmern verwenden diese einen integrierten Motor zur Erzeugung einer gerichteten Bewegung. Zu diesem Zweck erzeugen sie lokale Felder in ihrer Umgebung, die zu einem Antrieb des Schwimmers durch phoretische Kräfte führen. Da ihre Eigenschaften wie z.B. Wechselwirkungspotentiale, Geometrie, Dichteverteilung, Geschwindigkeit usw. individuell festgelegt werden können, sind sie als Modell für reale biologische Systeme perfekt geeignet. Abgesehen hiervon eignen sich künstliche Mikroschwimmer bereits zur Ausführung komplexer Aufgaben, wie z.B. dem Transport von Blutzellen oder anderen kleinen Objekten, was zukünftig den Wirkstofftransport von Medikamenten in der Medizin und Pharmazie ermöglichen könnte. In dieser Arbeit liegt der Fokus auf der Untersuchung der Eigenschaften von diffusiophoretischen Mikroschwimmern in Abhängigkeit ihrer geometrischen Form und ihrer Wechselwirkungen mit festen und dynamischen Objekten. Wir untersuchen zunächst die Bewegung asymmetrischer L-förmiger Teilchen experimentell und theoretisch. Durch ihre asymmetrische Form entsteht ein geschwindigkeitsabhängiges internes Drehmoment aufgrund der viskosen Reibung des Teilchens mit der Flüssigkeit. Dies führt zu einer kreisförmigen Bewegung mit konstantem Radius, die nur von den geometrischen Eigenschaften des Teilchens abhängt. Treffen solche Teilchen auf eine gerade Wand, kommt es in Abhängigkeit des Auftreffwinkels entweder zu einer stabilen Gleitbewegung oder einer Reflexion. Dies könnte Einblicke darüber geben, wie sich biologische Organismen durch enge Blutgefäße bewegen. In einem nächsten Schritt wenden wir L-förmige Teilchen zur Untersuchung der negativen Gravitaxis an, wie sie für zahlreiche biologische Mikroschwimmer beobachtet wird. Letztere haben die Fähigkeit, ihrer Sedimentation mittels einer geradlinigen Bewegung antiparallel zum Gravitationsvektor entgegenzuwirken. Jedoch ist es im Fall einiger Organismen noch immer Unklar, ob passive physikalische oder aktive physiologische Mechanismen für diesen Effekt verantwortlich sind. Wie anhand von Experimenten und numerischen Simulationen L-förmiger Teilchen mit homogener Massenverteilung demonstriert wird, ist die asymmetrische Form des Objekts ausreichend zur Erzeugung einer gravitaktischen Bewegung. Innerhalb eines Gravitationsfelds führt die Asymmetrie des Objekts zu einem rückstellenden Drehmoment, welches sein Antriebsdrehmoment kompensieren kann. Unter solchen Bedingungen ist der Schwimmer drehmomentfrei, was ihm eine geradlinige Bewegung nach oben ermöglicht. Passive Rückstellmechanismen asymmetrischer Teilchen in externen Feldern könnten zum Transport und zur Orientierungssynchronisation von Objekten verwendet werden. Der zweite Teil dieser Arbeit befasst sich mit kollektiven Phänomenen von aktiven Brown'schen Teilchen. Wir untersuchen experimentell und theoretisch die Wechselwirkung von aktiven und passiven Kolloiden gleicher Größe. Hierfür verwenden wir Lösungen passiver Teilchen, welche mit einer sehr kleinen Anzahl aktiver Kolloide versetzt werden. Bei geringen passiven Teilchendichten führt die Bewegung der aktiven Teilchen zur Bildung neuer oder der Kompression vorhandener kolloidaler Cluster. Falls die passive Teilchendichte weiter erhöht wird, zeigt das System bereits ohne eine aktive Teilchenbewegung bereits große kristalline Bereiche. Hier führt der Antrieb der aktiven Teilchen zu einem lokalen Schmelzen sowie zur Bildung von Defektlinien im Kristall. Das ständige Schmelzen und Rekristallisieren verursacht eine Verschiebung der Korngrenzen und das Wachstum vorhandener kristalliner Domänen. Dies könnte neue Möglichkeiten für die Produktion hochqualitativer kolloidaler Kristalle in der Industrie eröffnen.During the last decades, active Brownian motion of biological organisms received considerable interest by scientists of various fields. In contrary to Brownian particles, the motion of active Brownian objects is not in thermal equilibrium with its surroundings. They have the capability of taking up energy from their environment and converting it into directed motion. In case of biological swimmers this is realized by small, hair-like structures called flagella, which are attached to their body and set into motion by an integrated chemical motor. The high complexity of biological organisms can make research on specific properties a difficult task, because often a combination of many physical and physiological mechanisms, which cannot be separated from each other in experiments, contribute to one particular effect. To achieve a better understanding of such systems, scientists started creating autonomous artificial microswimmers with similar properties. Analog to biological swimmers, they use an integrated motor to create directed motion. For this purpose, they typically self-generate local field gradients in their environment, leading to a propulsion of the swimmer by phoretic forces. Since their properties as interaction potentials, geometry, density distribution, speed etc. are individually tunable, they are perfectly suitable as models for real biological systems. Apart from that, artificial microswimmers are already capable of performing complex tasks as e.g. cargo delivery, enabling future drug-delivery applications in pharmacy and medicine. In this thesis, the focus is on the investigation of the properties of self-diffusiophoretic microswimmers depending on their geometric shape and interactions with both fixed and dynamic objects. We first examine the motion of asymmetric L-shaped particles under bulk conditions both with experiments and in theory with two coupled Langevin equations. Their asymmetric shape gives rise to a velocity dependent internal torque leading originating from viscous forces of the fluid. This leads to a circular motion with a constant radius, which only depends on the particle's geometric properties. When such particles encounter a straight wall, depending on their angle of impact, they show either a stable sliding along the wall or a reflection. This could provide insight into how asymmetric biologic microorganisms move through narrow blood vessels or plant veins. In a next step, we apply L-shaped particles to study the properties of negative gravitaxis as it is observed for many biological microswimmers. The latter have the ability of counteracting sedimentation by a straight upward motion antiparallel to the gravitational vector. However, in case of many organisms it is still unclear, whether passive physical or active physiological mechanisms are accountable for this effect. As demonstrated by experiments and numerical simulations of L-shaped particles with homogeneous mass density, solely the asymmetric shape of a swimmer is sufficient to induce gravitactic motion. Inside a gravitational field, the asymmetry of the object results in a restoring torque which may compensate the one originating from the self-propulsion of the particle. Under such conditions, the swimmer is torque-free allowing it a straight upward motion. Passive alignment mechanisms of asymmetric particles in external fields could be used for transport purposes and their orientational synchronization. The second part of this thesis deals with collective phenomena of active Brownian particles. We experimentally and theoretically study the interaction of isometric active and passive spherical colloids. Therefore, suspensions of passive colloids with different densities are doped by a small amount of active particles. At small passive particle densities the motion of the actives leads to the formation of new or the compression of existing colloidal clusters. If the passive particle density is further increased, crystallization of the colloidal suspension occurs even in the absence of active motion. In this case, the propulsion of active particles leads to local melting and the creation of defect lines. The permanent melting and recrystallization causes a macroscopic shift of grain boundaries and the growth of existing crystalline domains. This could open new possibilities for the production of high quality colloidal crystals in industry

Formation, compression and surface melting of colloidal clusters by active particles

We demonstrate with experiments and numerical simulations that the structure and dynamics of a suspension of passive particles is strongly altered by adding a very small (<1%) number of active particles. With increasing passive particle density, we observe first the formation of dynamic clusters comprised of passive particles being surrounded by active particles, then the merging and compression of these clusters, and eventually the local melting of crystalline regions by enclosed active particles.publishe