Measurement and Characterization of Liquid Transfer between Two Solid Surfaces

  • Author / Creator
    Chen, Huanchen
  • Drop transfer from one solid surface to another through stretching liquid bridges between them is important for many industrial applications. Due to the different dominate forces, three possible regimes exist: Quasi-static Regime, where the transfer process is only dominated by surface forces; Dynamic Regime, where inertia and viscous forces are the dominate forces; and Transition Regime, where all three types of forces are important. In this dissertation, the transfer processes in all of the three different regimes are studied. For liquid transfer in Quasi-static Regime, the effects of contact angle hysteresis (CAH) are typically ignored in the literature. In this dissertation, with both the experimental measurements and simulation results from an analytical model, the importance of surface CAH in the transfer process is shown. Systematic studies on the role of advancing contact angle (θa), receding contact angle (θr) and CAH in determining the transfer ratio (α), maximum adhesion force (Fmax) and pull-off force (Fpf) are performed. The transfer ratio is found to be governed by contact line pinning at the end of the stretching stage caused by CAH, which is controlled by θr of the surfaces. An empirical equation which is able to predict the transfer ratio by only knowing θr of the two surfaces is provided. The value of Fmax is found to be strongly influenced by the contact line pinning in early stretching stage. For symmetric liquid bridge between two identical surfaces, Fmax may be determined only by θa, only by θr, or by both θa and θr, depending on the magnitude of the contact angles. For asymmetric bridges, Fmax is found to be affected by the length of the contact line pinning period on the two surfaces. For Fpf, it is found that when one of the surfaces has a θr larger than 90o, Fpf decreases with the increase of θr on either surface. For the cases where θr of both surfaces are smaller than 90o, significantly smaller Fpf is seen when contact line pinning occurs on both surfaces, as compared to Fpf when contact line pinning occurs only on one of the surfaces. For the liquid transfer in Transition and Dynamic Regimes, based on the value of Reynolds number (Re), the transfer process can have two different scenarios: one with negligible inertia effects (Re<<1) and the other with significant inertia effects (Re >1). For the liquid transfer with negligible inertia effects, the viscosity of the liquid is shown to act as a velocity shift such that, given the surface contact angle and the minimum separation between the two surfaces at which stretching begins, the transfer ratio is only a function of the Capillary number (Ca). Specifically, α converges from one plateau value to 0.5 with the increasing of Ca. The low-Ca plateau is the value in the Quasi-static Regime, whereas the high- Ca plateau of 0.5 is caused by the symmetric breakage shape of liquid bridge. Based on the observations, an empirical function α=F(Ca) was proposed and validated with experimental results. With this equation, by only knowing transfer ratio at two different stretching speeds in the Transition Regime, the transfer ratio under any value of stretching speed can be estimated. When the inertia effects become important (large Re), satellite drops appear when the liquid bridge breaks. Different from the transfer cases with negligible inertia effects, asymmetric shape of liquid bridge at the breakage is observed. Due to both the satellite drop and asymmetric breakage shape, α does not converge to 0.5 as it is observed in the system with negligible inertia effects.

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    Doctor of Philosophy
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.