We introduce a new model for contact angle saturation phenomenon in electrowetting on dielectric systems. This new model attributes contact angle saturation to repulsion between trapped charges on the cap and base surfaces of the droplet in the vicinity of the three-phase contact line, which prevents these surfaces from converging during contact angle reduction. This repulsion-based saturation is similar to repulsion between charges accumulated on the surfaces of conducting droplets which causes the well known Coulombic fission and Taylor cone formation phenomena. In our model, both the droplet and dielectric coating were treated as lossy dielectric media (i.e., having finite electrical conductivities and permittivities) contrary to the more common assumption of a perfectly conducting droplet and perfectly insulating dielectric. We used theoretical analysis and numerical simulations to find actual charge distribution on droplet surface, calculate repulsion energy, and minimize energy of the total system as a function of droplet contact angle. Resulting saturation curves were in good agreement with previously reported experimental results. We used this proposed model to predict effect of changing liquid properties, such as electrical conductivity, and system parameters, such as thickness of the dielectric layer, on the saturation angle, which also matched experimental results.

We introduce a new model for contact angle saturation phenomenon in electrowetting on dielectric systems. This new model attributes contact angle saturation to repulsion between trapped charges on the cap and base surfaces of the droplet in the vicinity of the three-phase contact line, which prevents these surfaces from converging during contact angle reduction. This repulsion-based saturation is similar to repulsion between charges accumulated on the surfaces of conducting droplets which causes the well known Coulombic fission and Taylor cone formation phenomena. In our model, both the droplet and dielectric coating were treated as lossy dielectric media (i.e., having finite electrical conductivities and permittivities) contrary to the more common assumption of a perfectly conducting droplet and perfectly insulating dielectric. We used theoretical analysis and numerical simulations to find actual charge distribution on droplet surface, calculate repulsion energy, and minimize energy of the total system as a function of droplet contact angle. Resulting saturation curves were in good agreement with previously reported experimental results. We used this proposed model to predict effect of changing liquid properties, such as electrical conductivity, and system parameters, such as thickness of the dielectric layer, on the saturation angle, which also matched experimental results.

We study rheotaxis of bull sperm inside microchannels to characterize the effects of flow and wall shape on sperm swimming behavior. We found that large percentage, 80 to 84%, of sperm cells exhibited positive rheotaxis (sperm cells swimming against the flow) within flow velocities of 33 to 134 µm/s. Sperm cells were also found to reverse their swimming direction when liquid flow direction was reversed. Time taken by sperm cells to reverse their swimming direction was inversely proportional to flow velocity. Sperm behavior was significantly affected by sperm position with respect to channel wall. Sperm cells close to channel wall moved upstream faster than sperm cells moving along channel centerline. Shear stress, which is an indicator of velocity distribution, was found to play an important role in regulating rheotactic behavior of sperm cells. Side pockets were added to some microchannels to mimic storage sites in mucosal folds and pockets in the fallopian tube of female reproductive system and sperm interaction with these pockets was monitored. We found that sperm cells tend to follow channel walls and enter these pockets without any chemical binding, which further confirms the wall tracking behavior of mammalian sperm cells. Our results confirm that sperm rheotaxis is a strong mechanism for guiding sperm cells to the oocyte along the female genital tracts.

We study rheotaxis of bull sperm inside microchannels to characterize the effects of flow and wall shape on sperm swimming behavior. We found that large percentage, 80 to 84%, of sperm cells exhibited positive rheotaxis (sperm cells swimming against the flow) within flow velocities of 33 to 134 µm/s. Sperm cells were also found to reverse their swimming direction when liquid flow direction was reversed. Time taken by sperm cells to reverse their swimming direction was inversely proportional to flow velocity. Sperm behavior was significantly affected by sperm position with respect to channel wall. Sperm cells close to channel wall moved upstream faster than sperm cells moving along channel centerline. Shear stress, which is an indicator of velocity distribution, was found to play an important role in regulating rheotactic behavior of sperm cells. Side pockets were added to some microchannels to mimic storage sites in mucosal folds and pockets in the fallopian tube of female reproductive system and sperm interaction with these pockets was monitored. We found that sperm cells tend to follow channel walls and enter these pockets without any chemical binding, which further confirms the wall tracking behavior of mammalian sperm cells. Our results confirm that sperm rheotaxis is a strong mechanism for guiding sperm cells to the oocyte along the female genital tracts.