No abstract

The response time for maximum drop deformation and its comparison with different time scales is established and verified with experiments. The applied fluctuation is achieved by applying a single wave perturbation of electrowetting with desired amplitude and frequency. To pinpoint the importance of the initial actuation conditions, the variance in the maximum drop deformation for a single wave perturbation is studied. The focus of this study was to analyze the maximum deformation of a drop for a wide range of actuation mechanism with a varied drop or surrounding medium viscosities. The drop response to this cyclic actuation is compared with the equivalent mass–spring–dampener system, and limitations of this approach are identified. Interestingly, the qualitative results were similar between the air and liquid medium cases, but the attainment of equilibrium configuration was dissimilar. As anticipated, the higher actuation magnitude and frequency deformed the drop significantly and thus altered the drop configuration. Higher viscosity of drops and the surrounding medium delayed the time to achieve the maximum deformation. Accurately predicting the time required for a drop to attain the maximum deformation is paramount for optimizing processes and based on microfluidics technology.

Merging of isolated liquid drops is a common phenomenon that may greatly be influenced by adding polymeric contents to the liquid. Here, we bring out an exclusive control on the dynamics of the intermediate liquid bridge, thus, formed via exploiting the interactions of an exciting electric field with a trace amount of polymeric inclusions present in the intermingling drops. Our results unveil a unique competition of the elastic recovery and time-oscillatory forcing during the drop-unification at early times. However, damped oscillations as a specific signature of the polymer concentration feature eventual stabilization of the bridge at later instants of time. We rationalize these experimental findings in light of a simple unified theory that holds its critical implications in droplet manipulation in a wide variety of applications encompassing digital microfluidics, chemical processing, and biomedical analytics.

Manipulating the coalescence of microdroplets has recently gained enormous attention in digital microfluidics and biological and chemical industries. Here, coalescence between two sessile droplets is induced by spreading them due to electrowetting. The electrocoalescence dynamics is investigated for a wide range of operating parameters such as electrowetting number, Ohnesorge number, driving frequency, and drop to surrounding medium viscosity ratio. Here, the characteristic time scale from the classical lubrication theory is modified with an additional driving and resisting force due to the electrostatic pressure force and liquid–liquid viscous dissipation, respectively. With the revised characteristic time scale, a universal bridge growth is shown between the two merging droplets following a 1/3 power law during early coalescence followed by a long-range linear variation. To ensure precise control on droplet coalescence, a geometric analysis is also performed to define the initial separation distance.

We report an electrode-embedded on-chip platform technology for the precise determination of ultra-short (of the orders of few nanoseconds) relaxation times of dilute polymer solutions, by deploying time-alternating electrical voltages....