Active surface tension driven micropump using droplet/meniscus pressure gradient

• Published in: Sensors and actuators. B, Chemical Chemical, 2013-04, Vol.180, p.114-121
• Main Authors: Shabani, Roxana, Cho, Hyoung J.
• Format: Article
• Language: English
• Subjects: Capacitance; Contact angle; Droplet; Droplets; electric field; Electric potential; Electrowetting on dielectric; equations; evaporation; hysteresis; Mathematical models; Micropump; Micropumps; Microvalve; Pressure gradients; shrinkage; Surface tension; Voltage
• ISSN: 0925-4005; 1873-3077
• DOI: 10.1016/j.snb.2012.05.058

An active micropump with a simple layout and no moving parts is designed and fabricated which has on demand flow on/off capability. The micropump is based on droplet/meniscus pressure gradient generated by electrowetting on dielectric (EWOD). By altering the contact angle between liquid and solid using an electric field a pressure gradient was induced and a small droplet was pumped into the channel via a uniform flow rate. A surface tension based propellant method was introduced as a low power consumption actuation method in microfluidic devices. The liquid contact angle on the EWOD substrate was measured vs. electric potential and was used to obtain the capacitance of the substrate by fitting Young–Lippmann's equation. The capacitance of the EWOD substrate was also calculated to be 10±0.6μF/m2 by measuring the dielectric layer thickness which showed excellent agreement with the former method. EWOD setup parameters such as capacitance, saturation contact angle, hysteresis contact angle and onset voltage were discussed. A coupled theoretical–experimental model was developed to predict how much voltage is needed to start the micropump for different droplet sizes. The modeling results revealed that for droplets with a radius smaller than 0.4mm the droplet will start going into the channel even when no voltage is applied. For any larger droplet, a certain voltage is needed to start the pump. It was also shown that decreasing the size of the input droplet and increasing the voltage will result in an increase in the pump flow. A model for describing the shrinkage of the micropump input droplet was developed, based on direct observations, which was in agreement with the forced wetting described in literature. This model was compared to the other models used to describe passively pumped droplets and evaporating microdrops.