Development of a Miniaturized Conveyor Driven by Electrowetting

In the world of (micro-)conveying systems, the pathway is mostly fixed, for example by the usage of tracks. Limited variations in the pathway can be achieved by installing switches, but a high flexibility in the trajectory is hard to achieve. The number of possible trajectories is usually fixed, and these are not easily reconfigurable. Friction also imposes limitations on the system, and causes wear to the transport system.

In this thesis, the design of a new miniaturized conveying system is discussed both theoretically and experimentally, in which the conveyor is transported by means of liquid droplets. Hereby, the drawbacks discussed in the previous paragraph, are mainly eliminated. The pathway is still partially fixed, but the electrodes allow for a larger design flexibility. This system consists of three main parts: a flat surface, containing the electrode array, on which the transportation takes place, four liquid droplets to support and transport the conveyor, and the conveyor itself. Each of these parts is discussed in this research.

The transportation is based on the principles of electrowetting. Electrowetting is a method to alter the surface tension of liquids, thus the extent to which the liquid wets the surface, by applying a voltage. This thesis discusses an actuation method, in which only a single plate, provided with electrodes, is required to apply the voltage. The electrodes are alternately grounded or actuated. Surface tension allows for a free-floating conveyor to be placed on the top side of the droplets, and to be transported when the droplets are actuated.

Secondly, this thesis presents a capacitance-based model to describe the droplet motion when a voltage is applied. The model thus allows to calculate equilibrium positions of the droplets, as function of, amongst others, the electrode shape, the droplet size and the distance between the electrodes. The experimental validation confirms the conclusions of the model: the equilibrium positions for different electrode shapes match the theoretically determined positions for various electrode shapes. Also the movement direction can be correctly predicted. A droplet speed of 4 mm/s is achieved with a voltage of 250V. 

As this movement model shows the importance of the droplet size for the movement, a geometric model is created to determine the shape of the liquid droplets that support and transport the platform. Also here, different factors play a role, amongst others the liquid volume and the load that is placed on top of the conveyor. Because the conveyor is only supported by the droplets, a larger load will cause a larger vertical compression of the droplets. Hereby, the contact area with the bottom plate is increased. To determine the maximal load that can be carried by the platform, also the distances between the liquid droplets and their distances to the platform edges need to be taken into account. Also this model is validated experimentally, showing a good correspondence with the theoretical values. 

As a last element in the conveyor system, also the conveyor itself is considered. Here, the focus lies on the pinning mechanism to fixate the droplets to a specific position on the platform. This allows to obtain a fixed kinimatic chain equivalent to a platform supported by four balls sliding over the base plate. 

Finally, also the full conveyor system is integrated and tested.