Circular Motion of Asymmetric Self-Propelling Particles

Abstract

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. [9] driving forces lead to active motion of micron-sized objects. So far, most studies have concentrated on spherical or rod-like microswimmers whose dynamics is well described by a persistent random walk with a transition from a short-time ballistic to a long-time diffusive behavior [10]. Such simple rotationally symmetric shapes, however, usually provide only a crude approximation for selfpropelling microorganisms, which are often asymmetric around their propulsion axis. Then, generically, a torque is induced that significantly perturbs the swimming path and results in a characteristic circular motion.In this Letter, we experimentally and theoretically study the motion of asymmetric self-propelled particles in a viscous liquid. We observe a pronounced circular motion whose curvature radius is independent of the propulsion strength but only depends on the shape of the swimmer. Based on the shape-dependent particle mobility matrix, we propose two coupled Langevin equations for the translational and rotational motion of the particles under an intrinsic force, which dictates the swimming velocity. The anisotropic particle shape then generates an additional velocity-dependent torque, in agreement with our measurements. Furthermore, we also investigate the motion of asymmetric particles in lateral confinement. In agreement with theoretical predictions we find either a stable sliding along the wall or a reflection, depending on the contact angle.Asymmetric L-shaped swimmers with arm lengths of 9 and 6 µm were fabricated from photoresist SU-8 by photolithography [11]. In short, a 2.5 µm thick layer of SU-8 is spin coated onto a silicon wafer, soft-baked for 80 s at 95• C and then exposed to ultraviolet light through a photo mask. After a post-exposure bake at 95• C for 140 s the entire wafer with the attached particles is coated with a 20 nm thick Au layer by thermal evaporation. When the wafer is tilted to approximately 90• relative to the evaporation source, the Au is selectively deposited at the front side of the short arms as schematically shown in Figs. 1(a),(b). Finally, the coated particles are released from the wafer by an ultrasonic bath treatment. A small amount of L-shaped particles is suspended in a homogeneous mixture of water and 2,6-lutidine at critical concentration (28.6 mass percent of lutidine), which is kept several degrees below its lower critical point (T C = 34.1• C) [12]. To confine the particle's motion to two dimensions, the suspension is contained in a sealed sample cell with 7 µm height. The particles ar...

Bistability and metabistability scenario in the dynamics of an ellipsoidal particle in a sheared viscoelastic fluid

“…is an unknown and needs to be calculated in order to satisfy the torque-free condition Eq. (8). Such a condition is imposed through constraints in each node of the particle surface by means of Lagrange multipliers [34].…”

“…For instance, elongated rigid particles are added into fluids to confer special final properties to the composite material [1][2][3][4]. Also, shape anisotropy of bioparticles such as bacteria [5][6][7], microswimmers [8], sperm cells [9], platelets [10] and hetero-cell aggregates [11] imparts peculiar dynamical properties to the overall flowing materials.…”

Dynamics and separation of circularly moving particles in asymmetrically patterned arrays

“…In other active matter systems, the particles undergo circular motion such as found in swimming bacteria near surfaces [28][29][30] and other types of swimming cells 31,32 . A variety of circle swimmers have been studied theoretically and in simulations [33][34][35][36] , and in recent experiments, it was demonstrated how to create artificial chiral colloidal moving particles that swim in circles with a fixed chirality depending on the asymmetric nature of the particle itself 37 . There are also proposals for creating molecular-sized chiral microswimmers by combining chiral molecules with chiral propellers 38 .…”

Ellipsoidal Brownian self-driven particles in a magnetic field

“…For example, when ABps interact among each other they may form clusters [6], thus these simple systems may shed light on the way active matter organizes, which eventually will lead to technology based on active materials [3,7]. In addition ABps can be considered as minimalistic models for studying swimming microorganisms [8,9]. Substantial efforts have also focused on the potential biomedical and environmental applications for these small selfdriven systems [10][11][12], such as targeted drug delivery, microsurgery, decontamination, environmental monitoring, among others [2].…”

“…Other related studies to the present work are: The theoretical and experimental study on the dynamics of a microswimmer with a L−shape made by Kummel et al [8], the dynamics of living active particles with a permanent magnetic moment (magnetotactic bacteria) in a rotating field treated by Cebers and coworkers [28,29], and the diffusive behavior of magnetotactic bacteria due to the random switching of the microorganisms flagellar motors reported in [30].…”