Fig. 2

Hydrodynamic analyses of the flows driven by PN-0, N-I, and PN-I microparticles. a Electrical device representations and equivalent circuit diagrams of different types of microparticles (in order of propulsive speed): PN-0, PN-II, N-I, and PN-I. b Tracer beads flowing around a PN-0 microparticle at a fixed field strength (E² = 54.4 kV² cm⁻²) and frequency (100 Hz). c The x- and y-velocity of tracer beads surrounding a PN-0 microparticle, indicating slow fluid flows. d Velocity of PN-0 microparticles as a function of the square of the electric field, indicating that their motion arises from weak-ICEP forces from the p-n junction. e Tracer beads flowing around an N-I microparticle at a fixed field strength (E² = 54.4 kV² cm⁻²) and frequency (100 Hz). f The x and y-velocity of tracer beads surrounding an N-I microparticle, indicating fluid flows with an intermediate speed. g Velocity of N-I microparticles as a function of the square of the electric field, indicating motions are due to ICEP forces from the Au contact. h Tracer beads flowing around a PN-I microparticle at a fixed field strength (E² = 54.4 kV² cm⁻²) and frequency (500 Hz). i The x and y-velocity of tracer beads surrounding a PN-I microparticle, indicating fast fluid flows. j Velocity of PN-I microparticles as a function of the square of the electric field, indicating motions are due to strong-ICEP forces via combined interactions from the p–n junction and the Au contact. Note: the propulsion characterization of the PN-II particles (or diode microparticles) is shown in Fig. 5. Each data point represents the average and SD (one above and one below for the error bars), as measured from five different microparticles in a single experiment. Scale bar =  20 μm