Figure 4: The microscopic images of a semen droplet at (A) 6.21 sec, (B) 7 sec, (C) 8.15 sec and (D) 8.51 sec. The voltage was applied at 4.5 sec on the left region of the images. We circled representative infertile sperms by blue circles and two fertile sperms by red circles. The induced flow of seminal plasma went to the left and then washed away the infertile sperms (blue circles). The fertile sperms (red circles) swam against the flow of seminal plasma.