Droplet, electrowetting on dielectric, micropump, hysteresis, dynamic contact angle, wetting line velocity, pinning, advancing and receding wetting lines, oils, phase equilibrium, probability, surface energy
In this dissertation physical phenomena relevant to (i) an interface formed between two fluids and a solid phase (wetting line) and (ii) an interface between three fluids (triple contact line) were investigated. In the former case, the wetting line (WL) phenomena which encompass the wetting line energy (WLE) or pinning, the wetting line velocity (WLV), and the contact angle hysteresis, were studied using a micropump based on electrowetting on dielectric (EWOD). In the latter case, the interfacial phenomena such as the air film lubrication effect and the liquid free surface deformation were taken into account to explain the dual equilibrium states of water droplets on liquid free surfaces. EWOD was implemented to devise a pumping method for a continuous flow in a microchannel. An active micropump with a simple layout and no moving parts is designed and fabricated which has on demand flow on/off capability. The micropump is based on droplet/meniscus pressure gradient generated by EWOD. By altering the contact angle between liquid and solid using an electric field a pressure gradient was induced and a small droplet was pumped into the channel via a uniform flow rate. A surface tension based propellant method was introduced as a low power consumption actuation method in microfluidic devices. For an initial droplet volume of 0.3µL and a power of 12nW a constant flow rate of 0.02µL/sec was demonstrated. Sample loading on-demand could be achieved by regulating an electric potential. Unexpectedly, the flow rate of the pump was found to be constant in spite of the changes in the droplet’s radius, which directly affects the pump’s driving pressure. iv The WL phenomena were studied in details to unravel the physical concept behind the micropump constant flow rate during the operation. An interesting observation was that the shrinking input droplet changes its shape in two modes in time sequence: (i) in the first mode its contact angle decreases while its wetting area remains constant due to the pinning, (ii) in the second mode the droplet’s WL starts to move while its contact angle changes as a function of its velocity. Contact angles were measured for the droplet advancing and receding WLs at different velocities to capture a full picture of contact angle behavior due to pinning and WLV effects. These results are also relevant to the meniscus inside the channel. The changes on the contact angle caused by the presence of EWOD at the bottom of the channel were studied in detail. The EWOD based micropump was used as a platform to study the contribution of the pinning and WLV effects on its constant flow rate. The effects of the WLE on the static contact angle and the WLV on the dynamic contact angle in the pump operation were investigated. Also the effect of EWOD voltage on the magnitude and uniformity of the micropump flow rate was studied. Dynamic contact angles (as a function of pinning and WLV) were used to accurately calculate the pressure gradient between the droplet and the meniscus and estimate the flow rate. It was shown that neglecting either of these effects not only results in a considerable gap between the predicted and the measured flow rates but also in an unphysical instability in the flow rate analysis. However, when the WLE and WLV effects were fully taken into account, an excellent agreement between the predicted and the measured flow rates was obtained. v For the study of the TCL between three fluids, aqueous droplets were formed at oil-air interface and two stable configurations of (i) non-coalescent droplet and (ii) cap/bead droplet were observed. General solutions for energy and force analysis were obtained and were shown to be in good agreement with the experimental observations. Further the energy barrier obtained for transition from configuration (i) to (ii), was correlated to the droplet release height and the probability of non-coalescent droplet formation. Droplets formed on the solid surfaces and on the free surface of immiscible liquids have various applications in droplet-based microfluidic devices. This research provides an insight into their formation and manipulation.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Mechanical and Aerospace Engineering
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dissertations, Academic -- Engineering and Computer Science, Engineering and Computer Science -- Dissertations, Academic
Shabani, Roxana, "Three-phase Contact Line Phenomena In Droplets On Solid And Liquid Surfaces: Electrocapillary, Pinning, Wetting Line Velocity Effect, And Free Liquid Surface Deformation" (2013). Electronic Theses and Dissertations. 2803.