Gravitaxis in Spherical Janus Swimming Devices

Abstract

In this work, we show that the asymmetrical distribution of mass at the surface of catalytic Janus swimmers results in the devices preferentially propelling themselves upward in a gravitational field. We demonstrate the existence of this gravitaxis phenomenon by observing the trajectories of fueled Janus swimmers, which generate thrust along a vector pointing away from their metallically coated half. We report that as the size of the spherical swimmer increases, the propulsive trajectories are no longer isotropic with respect to gravity, and they start to show a pronounced tendency to move in an upward direction. We suggest that this effect is due to the platinum caps asymmetric mass exerting an increasing influence on the azimuthal angle of the Janus sphere with size, biasing its orientation toward a configuration where the heavier propulsion generating surface faces down. This argument is supported by the good agreement we find between the experimentally observed azimuthal angle distribution for the Janus swimmers and predictions made by simple Boltzmann statistics. This gravitaxis phenomenon provides a mechanism to autonomously control and direct the motion of catalytic swimming devices and so enable a route to make autonomous transport devices and develop new separation, sensing, and controlled release applications.

A Pickering Emulsion Route to Swimming Active Janus Colloids

“…We focus specifically on spherical Janus colloids of the type shown in Figure 1 . This type of swimming device has been found to display a wide range of interesting colloidal phenomena, including autonomous guidance effects (gravitaxis,16 chemotaxis,17 and boundary steering),18 and has also been the subject of many currently untested theoretical proposals for high volume fraction collective phenomena 19. An advantage of Janus spheres as a system to explore emergent behavior is that they move without issuing bubbles and so their interactions via chemical “wakes” and hydrodynamics are amenable to being analyzed and experimentally observed, whereas nanotubes produce considerable convective flow due to bubble release 20.…”

Electrokinetic effects in catalytic platinum-insulator Janus swimmers

“…(c) Reaction rate as a function of average thickness of the Pt layer. Inset: a schematic view of the thickness profile of the Pt coating [37].…”

Preparation and 3D Tracking of Catalytic Swimming Devices

“…14 The particles produced here display relatively isotropic motion in all three dimensions; however if thicker platinum coatings, or larger particles are used, an upwards bias or gravitaxis can be imparted. 11 Details of the storage of the Janus colloids after manufacture may also effect the swimming speeds observed. The high surface energy clean platinum surface emerging from the evaporation stage is susceptible to surface contamination for example from hydrocarbons, and in particular thiols.…”

“…Plot a calibration curve to relate radius to z-position, and fit to an appropriate function (e.g., cubic equation) to allow interpolation. 11 2. Calibrate x,y axis 1.…”

“…This will aid future insights into motion characteristics for swimming devices. Evidence of this potential is given by the recent report of swimming devices being directed by gravity, 10,11 behavior which can most easily be visualized through the application of 3D tracking.…”