  We studied rotation of a disk propelled by a number of camphor pills
symmetrically distributed at its edge. The disk was put on a water surface so
that it could rotate around a vertical axis located at the disk center. In such
a system, the driving torque originates from surface tension difference
resulting from inhomogeneous surface concentration of camphor molecules
released from the pills. Here we investigated the dependence of the stationary
angular velocity on the disk radius and on the number of pills. The work
extends our previous study on a linear rotor propelled by two camphor pills
[Phys. Rev. E, 96, 012609 (2017)]. It was observed that the angular velocity
dropped to zero after a critical number of pills was exceeded. Such behavior
was confirmed by a numerical model of time evolution of the rotor. The model
predicts that, for a fixed friction coefficient, the speed of pills can be
accurately represented by a function of the linear number density of pills. We
also present bifurcation analysis of the conditions at which the transition
between a standing and a rotating disk appears.
