‘ I want to break free ’: Droplet trapping and release

Chair: Physics of Complex Fluids (PCF), University of Twente

‘ I want to break free ’: Droplet trapping and release

Microfluidics deals with the behavior, precise control, and manipulation of fluids that are geometrically constrained to a sub-millimeter scale. To control liquid droplets in a microfluidic chip many inventive techniques have been thought of. One of the more difficult aspects in droplet control is the holding and releasing of a droplet at a specific position for a certain amount of time. To accomplish this goal, we will create a chip that combines two existing techniques.

The first technique is described by Baroud et al.1 and is used to trap a droplet that has been confined between two plates. Small pinholes in the chip act as wells of surface energy, ensuring the droplet can be anchored to the pinhole. In this situation the balance between the anchoring force and the drag force on the droplet (due to the flow) determines whether the droplet remains trapped or will be released.

The second technique is electro-wetting (EW), which will be used to release the droplet from this trap. When a voltage is applied over an insulated electrode, the electric field changes the surface tensions of a water droplet on the substrate, effectively making the surface more hydrophilic. The droplet will thus prefer to increase its contact area with the electrode, and will spread in the direction of the electrode.

Now the balance between the anchoring force, the drag force, and the EW force will determine whether the droplet sticks or moves. The end result of this research project would be to find a regime where the setup will always trap a droplet, and where the release of the droplet can be completely determined by the activation of the EW electrode.

Figure 1: Above: a top-down image of a droplet held in place by a pinning hole against a mean external flow U and schematic side view.1

Right: the concept of EW. Applying a voltage over the electrode decreases the liquid-solid surface tension as shown by a decrease in contact angle.

The student will be challenged in many different subjects like chip design, experimental techniques and theoretical modeling. Since our group specializes in fluidics, the student will gain useful knowledge on for instance capillarity and wetting phenomena.






Arjen Pit, MSc.

Riëlle de Ruiter, MSc.







Project leader:

Dr. Michel Duits




1. Dangla, R., Lee, S. & Baroud, C.N. Trapping microfluidic drops in wells of surface energy. Phys Rev Lett 107, 124501 (2011).