University of Twente Student Theses


Suppression of coffee rings with electrowetting

Hoeven, Ivo van der and Colijn, Bram (2011) Suppression of coffee rings with electrowetting.

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Abstract:We investigated the relation between the ow speed inside droplets and the salt concentration diluted in the liquid of an AC electrowetted droplet. In another set of measurements we explored how the ow speed is related to the frequency of the applied voltage.The salt depency of this effect is a small part, but nevertheless it would be very usefull if the effects would be clear. This would mean that it would be possible to ajust the frequency on a droplet over time to account for evaporation and with that the in- crease of salt concentration. This way it might be possible to get a constant ow speed inside the droplet to make sure none of the particles deposit at the edges and a ho- mogeneous spot is created. This would give several useful implications for processes where its important to have an homogeneous spot of the diluted solid. In theory every drop should have an eigenfrequency dependent on the size of the droplet. This experiment was done to see what size a drop has to be for a frequency to have a maximum ow speed. In the fourth set of measurements, the effects on different concentrations of particles inside the solution was investigated. Nine different solutions were made with dif- ferent nanoparticle concentrations and the droplets were evaporated while measuring the contact angle. The last experiment was to evaporate droplets with different con- centrations of nano particles with an applied voltage with constant frequency. The goal of this experiment was to see when the pinning of the contact line of the droplet was suppressed and at what concentration it cannot be sup- pressed. In conclusion we did not observe any connections be- tween the salt concentration and the ow speed inside a droplet. Instead a periodic change in ow speed was ob- served dependent on the applied frequency. This is related to the resonant frequencies of the droplet. It is also de- pendent on de size of the droplet. This means, even at a fixed frequency, the ow speed can change because of the change in size as the droplet evaporates.
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics BSc (56962)
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