Contact line dynamics of droplets under electrowetting actuation
Vo, Xuan Quoc
Date of Issue2019-01-23
School of Mechanical and Aerospace Engineering
The dynamics of droplets spreading and retracting on solid substrates are both of fundamental and industrial interests. With the motivations to broaden our current knowledge on contact line dynamics as well as to provide technical guidelines for related applications, this thesis focuses on contact line dynamics of droplets spreading and retract- ing on solid substrates due to the electrocapillary eﬀects. First, in Chapter 4, contact line friction of electrowetting actuated droplets are investigated. The droplets are spread on the solid substrate using electrowetting actuation. Then, the voltage is released allowing the droplets to retract freely to study the retracting stages. The contact line friction co- eﬃcients of both the spreading and the retracting stages are found to depend equally on viscosities of the droplets and of the surrounding liquids. Therefore, it can be described by the geometric mean of the both viscosities. Based on the analysis of the contact line friction, a characteristic timescale for viscous droplets spreading and retracting on solid substrates is theoretically derived and experimentally veriﬁed. The subject of Chapter 5 is the transient behaviours of droplets under electrowetting actuation. These behaviours are categorised in either underdamped or overdamped regimes. Based on the force balance between driving forces and resisting forces of each regime, characteristic timescales for the transient dynamics of droplets for the two dynamical behaviours are established. The con- dition for the behaviours to change from one regime to the other is determined when the two timescales are comparable. Moreover, the actuation time, deﬁned as the duration for the droplets to reach new equilibrium after actuation, is studied. It is theoretically and experimentally shown that the minimisation of the actuation time is only achieved at the transition between the two regimes. Also, the dependence of the actuation time on system parameters such as viscosity, droplet radius and the applied voltage are investigated in detail. In Chapter 6, the dependence of static and dynamical behaviours of droplets on substrate properties is studied. It is shown that it is possible to bring the applied voltage to as low as the logic-signal-voltage level while maintaining suﬃcient electrowetting eﬀects. This investigation also reveals that dielectric materials have a minute eﬀect on the contact xivline friction coeﬃcient, therefore, insigniﬁcantly aﬀect to the transient dynamics of the actuated droplets. This result opens a great potential in numerous droplet-manipulating applications that requires low-voltage electrowetting actuation. Chapter 7 focuses on the detachment of droplets on solid substrates under electrowetting actuation. It is found that the detachment of droplets depends both on the hydrodynamical behaviours and contact line pinning of droplets. Particularly, droplets only can detach when the transient dy- namics is in the underdamped regimes and the surface energy diﬀerence created by the electrowetting eﬀect is higher than the sum of the contact line dissipation and an energy barrier created by the contact line pinning. Based on this, a theoretical model to pre- dict the condition for the detachment is developed. Moreover, the study about droplet detachment is extended to using maximum deformation actuation with over-saturation voltage. It is found that although the static contact angle saturates with over-saturation voltage, the deformation of droplets in the transient states are still facilitated by the over- saturation voltage. Therefore, it helps to increase the surface energy diﬀerence and to assist the detachment of the droplets. As a result, the detachable regime is expanded signiﬁcantly meaning that it is possible to detach droplets with smaller sizes and higher viscosity compared to non-saturation detaching methods. Finally, the initial velocity of de- tached droplets is investigated in detail. The initial detaching velocity can be characterised by the characteristic retracting velocity of the contact line. As a result, it is also can be predicted by using the characteristic timescales of the transient dynamics. In Chapter 8, contact line dynamics of droplets on dielectrowetting substrates is studied. In particular, this experiment focuses on investigating the spreading and retracting dynamics caused by the impinging of droplets on the dielectrowetting substrates. The dielectrowetting eﬀect is created by an application of an AC voltage to an array of electrodes beneath the dielec- tric layer of the substrate. It is found that the strength of the electric ﬁeld facilitates the spreading stage, whereas it has a negative eﬀect on the retracting stage. Interestingly, the retracting dynamics is asymmetrical, i.e., it retracts faster along the electrode’s direction compared to the electrode-perpendicular direction. A theoretical model determining the characteristic timescale of the asymmetrical retracting dynamic is proposed. The model is veriﬁed by the experimental data with a large range of the applied voltage. The presented results not only strengthen the current knowledge on contact line dynamics of droplets, but also provide important technical guidelines in design, control and optimisation of droplet manipulations using electrowetting. The results are also relevant in understanding other capillary driven phenomena such as wetting, droplet impact and evaporation.