ELECTRIC FIELD INDUCED DROPLET MANIPULATION

Abstract

In this thesis, we explore several droplet manipulation concepts on different length scales for a surface cleaning application. The design evolution to transfer these techniques from laboratory conditions to a chaotic environment, such as on the road, is an evolving engineering challenge where reliability and performance are equally important. Electrowetting and liquid dielectrophoresis are techniques by which an electromechanical response from an applied electric field enables precise droplet manipulation. This thesis presents several contributions to these technologies, focusing primarily on scalability, simplicity, and reliability.
The control of surface wettability using the electric methods attracts much attention due to their fast response (milliseconds), exceptional durability (hundreds of thousands of switching cycles) and low energy consumption (hundreds of microwatts). Furthermore, their superior performance and reliable nature have prompted a vast amount of literature to expand their application. They are widely used in several scientific and industrial fields, including microfluidics, optical devices, inject printing, energy harvesting, display technologies, and microfabrication.
Droplet actuation using electric methods has been a long-standing interest in microfluidics, and most often, it is limited by high operating voltages. The first actuation method explored in this thesis is based on interdigitated electrodes to generate a dielectrophoretic response. In order to apply an effective electrostatic force for droplet manipulation, the geometry of the electrodes must be optimised, which similarly leads to a lower operating voltage (as low as 30 V). Furthermore, microscale electrodes can be iteratively combined to realise larger arrays to move larger droplets. The iterative approach was developed for a large-scale device to manipulate droplets of varying sizes while keeping the actuation process simple.
In the second actuation method, a pair of microelectrodes separated by a variable gap distance generated an electrostatic gradient to produce a continuous droplet motion along the length of the electrode pad. The novel actuation method transported droplets of different sizes without active control. The droplet actuation was demonstrated on a larger scale using several platforms, including radial-symmetric, linear, and bilateral-symmetric droplet motion. An automated self-cleaning platform was tested in laboratory conditions and on the road. The technology has significant potential in the automotive sector to clean body parts, camera covers, and scanning sensors.
The electrostatic force applied across the droplet was calculated by placing a continuously moving droplet on a tilted platform and measuring the critical angle at which the droplet’s gravity overcomes the opposing applied electric force. Several electrode designs were also considered to evaluate the effect of electrode geometry on the actuation force. The droplet actuation was also modelled using an analytical approach to estimate the critical signal frequency, maximum electrostatic energy, and maximum electrostatic force.
Lastly, a tilting micromirror platform investigated the dielectrophoretic response without measuring the droplet contact angle. The mirror platform is also suitable for other optical applications as it provides three axes of movement for beam steering. The tilting platform enabled an angular coverage of up to 0.9° (± 0.02°), with a maximum displacement of 120 μm. We also explored the feasibility of using a microhydraulic actuator based on liquid dielectrophoresis for a microfluidic application. The actuation method opens new possibilities for positioning and manipulating particles and components. These could be hazardous medical materials or even radioactive substances, where direct contact should be avoided.