Measuring Attractive Interaction between a Self-Electrophoretic Micromotor and a Wall

Chemically driven micromotors exhibit a pronounced affinity for nearby surfaces, yet the quantification of this motor-wall interaction strength remains unexplored in experiments. Here, we apply an external force to a self-electrophoretic micromotor which slides along a wall and measures the force necessary to disengage the motor from the wall. Our experiments unveil that the required disengaging force increases with the strength of chemical driving, often surpassing both the motor's effective gravity and its propulsive thrust. Experimental results are reproduced by an electrokinetic numerical model that incorporates fully resolved double layers. The model delineates that the attractive force emerges from the accumulation of excessive protons between the motor and the wall, thereby exposing a nonequilibrium mechanism that engenders attractive interactions between objects of like charge.