(560d) Dispensing Surfactant-Containing Water Droplets Using Electrowetting

Many droplet-based microfluidic schemes use the electrowetting-on-dielectric (EWOD) mechanism to dispense and transport droplets of conductive liquids. In particular, the addition of surfactant to water has garnered interest in recent years. For the biomedical field, surfactant can be used to prevent adsorption of biological cells and proteins to solid surfaces.¹ For the field of fusion energy, the ability to dispense precise volumes of oil and water-surfactant droplets (containing reactive chemicals) could be used to make fuel pellets.²

Surfactants create serious impediments, unfortunately, to the droplet-dispensing process. Sometimes, at lower voltages, the droplets cannot be separated from the parent reservoir of liquid. A persistent membrane of fluid that forms between the reservoir and the droplet, spanning the top and bottom electrode surfaces, creates a problem for dispensing surfactant-laden water droplets. As the surfactant concentration approaches the critical micelle concentration (CMC), rupturing this membrane becomes increasingly difficult. The persistence of the membrane is related to the reduced liquid/air surface tension, the lower liquid/substrate surface energy, and the higher disjoining pressure within the membrane.³ Voltages higher than 75 V_rms at 10 kHz can be used to promote the dispensing process, resulting in a greatly increased likelihood of serious damage to the dielectric coatings.

The membrane-rupture problem is resolved by embedding a thin resistive heater element into the lab-on-chip device to assist the EWOD forces in creating droplets. The heater strip is designed to bisect the membrane and, when a dc current is applied to the heater strip, the membrane destabilizes within ~0.5 s. The rupture does not seem to occur at the site of the narrow heater strip; rather, the membrane seems to become destabilized along the entire length of the membrane, forming a pearl chain of droplets residing between the top and bottom electrodes. We believe that a transient thermocapillary mechanism,⁴ driven by the imposed temperature gradient, is the cause of the observed rupture. Using the resistive heater, we are able to dispense aqueous droplets of volume from ~0.5 to ~12.5 µL, containing surfactant Silwet L-77 at concentrations up to ~5x CMC. Under a broad range of conditions, reproducibility of the droplet volumes is dramatically improved, compared to the case having no resistive heater. Furthermore, the required EWOD voltage is maintained in the range of 75 V_rms at 10 kHz. The voltage applied to the resistive heater is 10 to 30 V_dc.

This work was supported by the Frank J. Horton Graduate Fellowship Program. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.

References

1. G. J. Shah, J. L. Veale, Y. Korin, E. F. Reed, H. A. Gritsch, and C.-J. Kim, Biomicrofluidics 4, 044106 (2010); E. M. Miller, A. H. C. Ng, U. Uddayasankar, and A. R. Wheeler, Anal. Bioanal. Chem. 399, 337 (2010).

2. W. Wang, Ph.D. thesis, University of Rochester, 2012.

3. D. S. Ivanova and J. K. Angarska, Colloid Surf. A, Physiochem. Eng. Aspect 438, 93 (2013).

4. L. Y. Yeo, R. V. Craster, and O. K. Matar, Phys. Rev. E 67, 056315 (2003).