Mechanism of locomotion of synthetic nanomotors in a viscous fluid

A mechanism of nanomotor locomotion in a surrounding viscous fluid containing charged particles is considered. In contrast to a mechanism proposed in the literature, according to which nanomotor locomotion is induced by a concentration gradient of certain type particles produced by asymmetric chemical or electrochemical reactions occurring on the nanomotor surface, we hypothesize that nanomotor locomotion can be driven by hydrodynamic interactions in the case of identical concentrations of different-sized ions. To justify the hypothesis, the dynamics of a nanomotor surrounded by a viscous fluid is studied using the diffusion model of electrohydrodynamics and, additionally, the model of a dipolar aggregate surrounded by a cloud of equally but oppositely charged fine particles of different sizes is considered. It is assumed that the total charge of all fine particles is zero and the oppositely charged particles have identical concentrations in the ambient fluid. Computations have confirmed that the nanomotor can move in this case. The direction and speed of the motion depend substantially on both the distribution of the particles in the surrounding fluid and on their sizes. Symmetry breaking in the particle distribution gives rise to a velocity component perpendicular to the dipolar moment direction. In the case of the chemical or electrochemical mechanism of ion formation, symmetry breaking in the ion distribution can be caused by symmetry violations in the nanomotor shape or by possible impurities participating in the reaction, so, to control the nanomotor motion, an external field orienting the nanomotor in the prescribed direction has to be applied. The proposed mechanism of nanomotor locomotion can be used to control mass transfer in colloidal suspensions.