Numerical and theoretical analyses of the dynamics of droplets driven by electrowetting on dielectric in a Hele-Shaw cell

Significance 

Microfluidic devices are used in various applications such as DNA microarrays, optical display, and drug screening. Generally, surface tension modulation is used in these devices because it is dominant over body forces and surface forces thus enabling efficient control of liquids. The use of surface tension has also been extended to controlling wettability. The demand for efficient microfluidic devices has attracted researchers to develop various methods for predicting the shape and speed of droplets. As such, a computational fluid dynamics approach has been identified as a promising alternative.

In the previously published literature, two-dimensional simulation has been fruitfully used to predict droplet dynamics. For instance, Hele-Shaw approximation utilized the pressure gradient to determine the two-dimensional velocity filed and pressure boundary condition to determine the effect of wettability. Consequently, electrowetting on dielectric simulations failed to produce necessary splitting or splitting that were too fast. Therefore, the need to accurately represent wettability has led to three-dimensional simulation other than two-dimensional. However, a three-dimensional simulation of an electrowetting process taking into consideration the full dynamic contact angle has not been fully explored.

Recently, Professor. Yasufumi Yamamoto at Kansai University in collaboration with Dr. Takahiro Ito at Nagoya University, Dr.Tatsuro Wakimoto and Professor Kenji Katoh at Osaka City University investigated numerically and theoretically the droplet movement by electrowetting on a dielectric in a Hele-Shaw cell. They developed a three-dimensional simulation to predict the speed and shape of the droplets and compare them to those obtained experimentally. Also, they expected to estimate the timescale for changes in the contact angle. Their work is published in Journal of Fluid Mechanics.

The research team developed the simulation method by representing electrowetting on dielectric using the Young-Lippmann equation while the moving contact line was represented by a generalized Navier boundary condition and front trackingmethod. Furthermore, they proposed a theoretical model for the droplet motion which was used to determine the voltage dependency on the droplet speed.

The authors observed that the developed numerical simulations were capable of reproducing the speed and shape evolution of droplets obtained experimentally. Also, they described the effects of various values of voltage and shape parameters on the droplet motion. Furthermore, the time scale for the change in the contact angle was estimated successfully by observing the behavior of the contact line at the start of the droplet motion. Both the simulation results and the experimental results agreed well thus signifying the accuracy of the developed models.

The study will, therefore, advance the design, optimization, and operation of microfluidic devices. This is because the developed theoretical model overcomes most of the drawbacks experienced with the previously used models. For instance, it enables accurate and precise timescale estimations which is a key consideration in the design and operation of microfluidic devices. This can be attributed to the full three-dimensional numerical simulation which takes advantage of the complex droplet deformation near the electrode boundaries.

Reference

Yamamoto, Y., Ito, T., Wakimoto, T., & Katoh, K. (2018). Numerical and theoretical analyses of the dynamics of droplets driven by electrowetting on dielectric in a Hele-Shaw cell. . Journal of Fluid Mechanics, 839, 468-488.

Go To Journal of Fluid Mechanics

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