“This paper shows how surface tension can be effectively used in controlling a small-scale fluid flow such as a droplet motion in a 10 micron-sized channel”

In my view, this paper is highly cited because of its pioneering role in using surface tension as a driving force for a microscale liquid flow. Surface tension emerges as one of the major forces when the size of interest diminishes below millimeter scale. This paper presents scientific backgrounds and practical tools for realizing microfluidic devices driven by electrically-controlled surface tension. Among the issues that can be answered using information provided in this paper are the following:

• What would be the most efficient way to control the surface tension?
• How can a device be designed based on the fundamental principles suggested?
• What is underlying the understanding and interpretation?
• How can the microfluidic device be fabricated and tested?

The readers would also find useful analyses and toolsets for their own applications that can cover a broad spectrum of microscale engineering and scientific problems.

This paper describes a unique way of understanding the surface-tension-driven microfluidic flow controlled by an electric field. Surface tension is an energy phenomena occurring at the interface between different phases. When an electric potential is applied between the phases, the surface tension, surface energy per unit area, will change to balance the electrical energy. An energy principle is used to explain the fluid behavior due to surface tension variation as the electrical potential varies. A striking conclusion was made as to the pressure of the fluid flow by electrically controlled surface tension. It was found that the resulting pressure of flow could be interpreted as the one that could be obtained with comb-drive-type electrostatic actuation, a popular micro-actuation mechanism.

This paper shows how surface tension can be effectively used in controlling a small-scale fluid flow such as a droplet motion in a 10 micron-sized channel. A surface tension force does not easily draw attention in our daily life because other types of forces such as gravity and mass are more significant. No one suffers from the sticking force by surface tension when walking on a wet road. However, when we look into Mother Nature a little bit closer, especially under a microscope, a remarkable new world is open to us that is governed by surface tension. A water strider, for example, can float on a pond with its four legs supported entirely by the force of surface tension. There is a scaling law behind this upside-down physics which needs to be applied differently in different scales. Human-made micro-devices also suffer from this "gigantic" force of surface tension which, in many cases, causes trouble in operating them in a small, sticky world. We can, on the contrary, control and use the surface tension for some useful targets such as producing a fluid flow in a tiny channel. This approach would be far more powerful than using pumps and valves in a microchannel. This paper describes how an electrical energy could be effectively used to do this job of controlling and using surface tension.

I was a graduate student at UCLA when I first became involved in this research. In my previous research, I had developed a micro-machined device that could produce a quite impressive motion of a mercury droplet in an electrolyte, driven by electrically controlled surface tension. Even though there were apparently several applications that could benefit from the microfluidic-actuation-mechanism suggested in the previous research, I found more substantial applications could be impacted if an aqueous liquid can be directly driven droplet by droplet using the principle used in this paper. Applications already being demonstrated nowadays include massively parallel, droplet-based mixing and separation of various chemical/bio-liquids and optical devices.