Electrowetting based local sensing of liquid properties using relaxation dynamics of stretched liquid interface

Abstract

Hypothesis: Monitoring progression of biochemical processes is required for medical and industrial applications. Spatiotemporal changes in fluid properties can be measured to determine progress of biochemical processes like blood coagulation. Localised electrowetting-on-dielectric (EWOD) actuates a part of droplet contact line, allowing local measurement of fluid properties without inducing bulk fluid motion, which is unlike full droplet oscillation-based techniques.

Experiments: In this work, narrow electrodes (50-450 μm) were used to actuate a portion of drop interface. Dynamics of interface actuation and relaxation was used to estimate the local visco-elastic properties of the droplet.

Findings: For local interface motion, theory predicts a generic dispersion relation ω=cqⁿ. In agreement with theory, decay time was found to be proportional to viscosity and inversely proportional to surface tension. Interface displacement remained almost constant for different viscosities, but it decreased with increase in surface tension. Capability to measure spatiotemporal dynamics of chemical process was demonstrated for sugar dissolution in a droplet of water. For full droplet oscillation-based techniques, the induced bulk flows adversely affect the monitored process. Localised EWOD reduces bulk flows in the sample. So, this technique was applied to study blood coagulation dynamics, enlightening the future prospect of developing biomedical sensors.

Keywords: Contact line movement; Decay time; Electrowetting; Interface dynamics; Localised sensing.