Review on Active Matter published in Rev. Mod. Phys.

Active Brownian particles in complex and crowded environments

Active Brownian particles in complex and crowded environments (Invited review)
Clemens Bechinger, Roberto Di Leonardo, Hartmut Löwen, Charles Reichhardt, Giorgio Volpe & Giovanni Volpe
Reviews of Modern Physics 88(4), 045006 (2016)
DOI: 10.1103/RevModPhys.88.045006
arXiv: 1602.00081

Differently from passive Brownian particles, active particles, also known as self-propelled Brownian particles or microswimmers and nanoswimmers, are capable of taking up energy from their environment and converting it into directed motion. Because of this constant flow of energy, their behavior can be explained and understood only within the framework of nonequilibrium physics. In the biological realm, many cells perform directed motion, for example, as a way to browse for nutrients or to avoid toxins. Inspired by these motile microorganisms, researchers have been developing artificial particles that feature similar swimming behaviors based on different mechanisms. These man-made micromachines and nanomachines hold a great potential as autonomous agents for health care, sustainability, and security applications. With a focus on the basic physical features of the interactions of self-propelled Brownian particles with a crowded and complex environment, this comprehensive review will provide a guided tour through its basic principles, the development of artificial self-propelling microparticles and nanoparticles, and their application to the study of nonequilibrium phenomena, as well as the open challenges that the field is currently facing.

Influence of Active Particles on Colloidal Clusters published in Soft Matter

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

Formation, compression and surface melting of colloidal clusters by active particles
Felix Kümmel, Parmida Shabestari, Celia Lozano, Giovanni Volpe & Clemens Bechinger
Soft Matter 11(31), 6187—6191 (2015)
DOI: 10.1039/C5SM00827A

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 (o1%) 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.

Reply to Comment on Circular Microswimmers published in Phys. Rev. Lett.

Reply to comment on “Circular motion of asymmetric self-propelling particles”

Reply to comment on “Circular motion of asymmetric self-propelling particles”
Felix Kümmel, Borge ten Hagen, Raphael Wittkowski, Daisuke Takagi, Ivo Buttinoni, Ralf Eichhorn, Giovanni Volpe, Hartmut Löwen & Clemens Bechinger
Physical Review Letters 113(2), 029802 (2014)
DOI: 10.1103/PhysRevLett.113.029802
arXiv: 1407.4016

See also “Circular motion of asymmetric self-propelling particles”, Physical Review Letters 113(2), 029802 (2014)

Circular Microswimmers published in Phys. Rev. Lett.

Circular motion of asymmetric self-propelling particles

Circular motion of asymmetric self-propelling particles
Felix Kümmel, Borge ten Hagen, Raphael Wittkowski, Ivo Buttinoni, Giovanni Volpe, Hartmut Löwen & Clemens Bechinger
Physical Review Letters 110(19), 198302 (2013)
DOI: 10.1103/PhysRevLett.110.198302
arXiv: 1302.5787

See also Reply to comment on “Circular motion of asymmetric self-propelling particles”, Physical Review Letters 113(2), 029802 (2014)

Micron-sized self-propelled (active) particles can be considered as model systems for characterizing more complex biological organisms like swimming bacteria or motile cells. We produce asymmetric microswimmers by soft lithography and study their circular motion on a substrate and near channel boundaries. Our experimental observations are in full agreement with a theory of Brownian dynamics for asymmetric self-propelled particles, which couples their translational and orientational motion.

Featured in “Synopsis: Round and Round in Circles”, Physics (May 9, 2013)

Active Brownian Motion Tunable by Light published in J. Phys. Condens. Matter

Active Brownian motion tunable by light

Active Brownian motion tunable by light
Ivo Buttinoni, Giovanni Volpe, Felix Kümmel, Giorgio Volpe & Clemens Bechinger
Journal of Physics: Condensed Matter 24(28), 284129 (2012)
DOI: 10.1088/0953-8984/24/28/284129
arXiv: 1110.2202

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 paper, we investigate in more detail 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 behavior of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent.

Microswimmers in Patterned Environments published in Soft Matter

Microswimmers in patterned environments

Microswimmers in patterned environments
Giovanni Volpe, Ivo Buttinoni, Dominik Vogt, Hans-Jürgen Kümmerer & Clemens Bechinger
Soft Matter 7(19), 8810—8815 (2011)
DOI: 10.1039/C1SM05960B
arXiv: 1104.3203

Tiny self-propelled swimmers capable of autonomous navigation through complex environments provide appealing opportunities for localization, pick-up and delivery of microscopic and nanoscopic objects. Inspired by motile cells and bacteria, man-made microswimmers have been created and their motion in homogeneous environments has been studied. As a first step towards more realistic conditions under which such microswimmers will be employed, here we study, experimentally and with numerical simulations, their behavior in patterned surroundings that present complex spatial features where frequent encounters with obstacles become important. To study the microswimmers as a function of their swimming behavior, we develop a novel species of microswimmers whose active motion is due to the local demixing of a critical binary liquid mixture and can be easily tuned by illumination. We show that, when microswimmers are confined to a single pore whose diameter is comparable with their swimming length, the probability of finding them at the confinement walls significantly increases compared to Brownian particles. Furthermore, in the presence of an array of periodically arranged obstacles, microswimmers can steer even perpendicularly to an applied force. Since such behavior is very sensitive to the details of their specific swimming style, it can be employed to develop advanced sorting, classification and dialysis techniques.

Reply to Comment on Influence of Noise on Force Measurements published in Phys. Rev. Lett.

Reply to comment on “Influence of noise on force measurements”

Reply to comment on “Influence of noise on force measurements”
Giovanni Volpe, Laurent Helden, Thomas Brettschneider, Jan Wehr & Clemens Bechinger
Physical Review Letters 107(7), 078902 (2011)
DOI: 10.1103/PhysRevLett.107.078902
arXiv: 1101.3916

See also “Influence of noise on force measurements”, Physical Review Letters 104(17), 170602 (2010)

Comparison Between Force Measurement Methods published in Phys. Rev. E

Force measurement in the presence of Brownian noise: arXiv:1009.2386
Equilibrium distribution method vs. drift method

Force measurement in the presence of Brownian noise: Equilibrium distribution method vs. drift method
Thomas Brettschneider, Giovanni Volpe, Laurent Helden, Jan Wehr & Clemens Bechinger
Physical Review E 83(4), 041113 (2011)
DOI: 10.1103/PhysRevE.83.041113
arXiv: 1009.2386

The study of microsystems and the development of nanotechnologies require alternative techniques to measure piconewton and femtonewton forces at microscopic and nanoscopic scales. Among the challenges is the need to deal with the ineluctable thermal noise, which, in the typical experimental situation of a spatial diffusion gradient, causes a spurious drift. This leads to a correction term when forces are estimated from drift measurements [G. Volpe, L. Helden, T. Brettschneider, J. Wehr, and C. Bechinger, Phys. Rev. Lett. 104, 170602 (2010)]. Here we provide a systematic study of such an effect by comparing the forces acting on various Brownian particles derived from equilibrium-distribution and drift measurements. We discuss the physical origin of the correction term, its dependence on wall distance and particle radius, and its relation to the convention used to solve the respective stochastic integrals. Such a correction term becomes more significant for smaller particles and is predicted to be on the order of several piconewtons for particles the size of a biomolecule.

Influence of Noise on Force Measurements published in Phys. Rev. Lett.

Influence of noise on force measurements

Influence of noise on force measurements
Giovanni Volpe, Laurent Helden, Thomas Brettschneider, Jan Wehr & Clemens Bechinger
Physical Review Letters 104(17), 170602 (2010)
DOI: 10.1103/PhysRevLett.104.170602
arXiv:  1004.0874

See also Reply to comment on “Influence of noise on force measurements”, Physical Review Letters 107(7), 078902 (2011)

We demonstrate how the ineluctable presence of thermal noise alters the measurement of forces acting on microscopic and nanoscopic objects. We quantify this effect exemplarily for a Brownian particle near a wall subjected to gravitational and electrostatic forces. Our results demonstrate that the force-measurement process is prone to artifacts if the noise is not correctly taken into account.

Enhanced TIRM published in Opt. Express

Novel perspectives for the application of total internal reflection microscopy

Novel perspectives for the application of total internal reflection microscopy
Giovanni Volpe, Thomas Brettschneider, Laurent Helden & Clemens Bechinger
Optics Express 17(26), 23975—23985 (2009)
DOI: 10.1364/OE.17.023975
arXiv: 0909.5131

Total Internal Reflection Microscopy (TIRM) is a sensitive non-invasive technique to measure the interaction potentials between a colloidal particle and a wall with femtonewton resolution. The equilibrium distribution of the particle-wall separation distance z is sampled monitoring the intensity I scattered by the Brownian particle under evanescent illumination. Central to the data analysis is the knowledge of the relation between I and the corresponding z, which typically must be known a priori. This poses considerable constraints to the experimental conditions where TIRM can be applied (short penetration depth of the evanescent wave, transparent surfaces). Here, we introduce a method to experimentally determine I(z) by relying only on the distance-dependent particle-wall hydrodynamic interactions. We demonstrate that this method largely extends the range of conditions accessible with TIRM, and even allows measurements on highly reflecting gold surfaces where multiple reflections lead to a complex I(z).