Gravitaxis in Spherical Janus Swimming Devices
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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