Helical paths, gravitaxis, and separation phenomena for mass-anisotropic self-propelling colloids: Experiment versus theory

Abstract

The self-propulsion mechanism of active colloidal particles often generates not only translational but also rotational motion. For particles with an anisotropic mass density under gravity, the motion is usually influenced by a downwards oriented force and an aligning torque. Here we study the trajectories of self-propelled bottom-heavy Janus particles in three spatial dimensions both in experiments and by theory. For a sufficiently large mass anisotropy, the particles typically move along helical trajectories whose axis is oriented either parallel or antiparallel to the direction of gravity (i.e., they show gravitaxis). In contrast, if the mass anisotropy is small and rotational diffusion is dominant, gravitational alignment of the trajectories is not possible. Furthermore, the trajectories depend on the angular self-propulsion velocity of the particles. If this component of the active motion is strong and rotates the direction of translational self-propulsion of the particles, their trajectories have many loops, whereas elongated swimming paths occur if the angular self-propulsion is weak. We show that the observed gravitational alignment mechanism and the dependence of the trajectory shape on the angular self-propulsion can be used to separate active colloidal particles with respect to their mass anisotropy and angular self-propulsion, respectively.

Helical and oscillatory microswimmer motility statistics from differential dynamic microscopy

“…As is clear from figure 5(b), the mean along-helix speed value grows with the helix radius R. The functional form for this variation, provided by equation (10), is explicitly plotted in figure 5(d) and compared with the ratio between the high q value of the mean speed from fits and the simulation input. Similarly, the theoretical and fitted ratios of the corresponding standard deviations, using equation (11) are plotted on the same graph. The speed and standard deviation data follow the expected variation with R, which provides a useful check that the dependencies predicted by our analytical theory are borne out by the simulated experiments.…”

“…Micro-organisms and artificial swimmers typically are observed to swim along trajectories of a helical nature [10][11][12]. The helical motion arises from body rotation about an axis that differs from the swimming direction [11,13] and is the expected outcome of systems that lack perfect symmetry.…”

“…Micro-organisms and artificial swimmers typically are observed to swim along trajectories of a helical nature [10][11][12]. The helical motion arises from body rotation about an axis that differs from the swimming direction [11,13] and is the expected outcome of systems that lack perfect symmetry. For biological swimmers such as microalgae [10,14] or spermatozoa [12] the rotation originates from torques caused by non-planar flagellar motion [13,15].…”

“…In the case of microalgae, the rotation is observed to have a biological role in that it allows cells to sample the light environment and move towards regions that are photosynthetically optimal (phototaxis) [16,17]. For artificial microswimmers such as catalytic Janus particles the rotation is likely due to a combination of body and coating imperfections [11].…”

“…Single-particle tracking [18] and ensemble-averaged techniques, such as dynamic light scattering (DLS) [19], have been until recently the main techniques used to probe the spatio-temporal dynamics of particle suspensions. Particle tracking in video microscopy, developed for the characterisation of passive colloidal dynamics [18], has been widely applied to study active systems, biological [15,20] and synthetic [11,21]. However, the characterisation of three-dimensional motion, such as helical swimming, with standard imaging microscopy is limited by the tracking depth of the microscope [18].…”