There are two outstanding problems associated with the contact line. One is the moving contact line (MCL) problem, as it is incompatible with the non-slip boundary condition and would lead to unphysical infinite dissipation. The other is contact angle hysteresis (CAH), where the contact line is pinned by the physical roughness and/or chemical heterogeneities on the solid surface, which causes the dynamic contact angle between the liquid interface and solid surface to depend on the direction of fluid motion. In this thesis work, a systematic study of the contact line dissipation and wetting dynamics is carried out using a newly developed hanging fiber probe. The aims of the thesis work are (i) to study the friction coefficient of a fluctuating contact line, (ii) to further unde...[ Read more ]

There are two outstanding problems associated with the contact line. One is the moving contact line (MCL) problem, as it is incompatible with the non-slip boundary condition and would lead to unphysical infinite dissipation. The other is contact angle hysteresis (CAH), where the contact line is pinned by the physical roughness and/or chemical heterogeneities on the solid surface, which causes the dynamic contact angle between the liquid interface and solid surface to depend on the direction of fluid motion. In this thesis work, a systematic study of the contact line dissipation and wetting dynamics is carried out using a newly developed hanging fiber probe. The aims of the thesis work are (i) to study the friction coefficient of a fluctuating contact line, (ii) to further understand contact angle hysteresis and (iii) to investigate the wetting behaviour of lipid monolayers on a thin fiber surface under different surface conditions.

In the experiment, we develop an atomic force microscopy (AFM) based hanging fiber probe with one end glued to an AFM cantilever beam and the other end in contact with a liquid-air interface. The hanging fiber probe is used to directly measure minute changes in the viscous damping resulting from the liquid, which the hanging fiber is immersed into. The motion of the hanging fiber probe is found to be accurately described by the Langevin equation for a damped harmonic oscillator, from which the friction coefficient ξ of the hanging fiber is obtained. It is found that ξ increases almost linearly with the immersion length h and a non-zero intercept is observed due to the dissipation of the contact line between the liquid-air interface and the fiber surface. For a fix value of h, the measured ξ is directly linked to the liquid viscosity η, which is found to be accurate compared to the independently measured values using a rheometer.

With this well tested hanging fiber probe, we measure the friction coefficient ξ_c of a fluctuating (and slipping) contact line formed at a liquid-air interface and in a thin soap film. By measuring the broadening of the resonant peak of the cantilever system with varying liquid viscosity η, we obtain a universal scaling law of the contact line friction coefficient, ξ_c = απdη, for various liquids ( and soap films) of viscosity η and glass fibers of diameter d with different surface treatments. The proportionality constant α is found to be α = 0.8 ± 0.2, independent of the contact angle and surface tension. The obtained scaling law is further supported by the numerical simulations based on a phase field model under the generalized Navier boundary conditions.

In addition, a dual-mode hanging fiber probe is developed by assembling a hanging fiber with a proper amount of glue and a particular aspect ratio. In addition to the normal vertical oscillation mode, a horizontal oscillation mode is simultaneously detected in the power spectrum. The experiment verifies the prediction of the linear elasticity theory and further confirms the scaling law of the contact line friction coefficient ξ_c.

By directly measuring the capillary force acting on the contact line, we study the wetting dynamics and contact angle hysteresis of DPPC monolayers using the hanging fiber probe. In the experiment, three types of fiber surfaces with agarose-, DPPC- and FTS-coatings are used. A transition from partial to complete wetting is found for the DPPC-coated and FTS-coated fiber surfaces, when the surface tension of the DPPC monolayer becomes smaller than a critical value γ_c. The measured values of the capillary force are found to be accurate compared with those obtained using a Wilhelmy plate. This new apparatus is also used to measure the height of the capillary rise and the results agree very well with the theory prediction made by de Gennes el al. An additional precursor film is found for complete wetting liquids with zero contact angle.

The AFM system is also used to study contact angle hysteresis for the DPPC monolayers on DPPC-coated and FTS-coated fibers. By directly measuring the capillary force, we find a universal behaviour of the hysteresis force f_h as a function of the (equilibrium) contact angle θ_e with f_h ≃ -2γsinθ_e(sinδθ)_c and the proportionality constant (sinδθ)_c is found to depend only on the physical roughness of the solid surface. In addition, the effective spring constant k of the stretched DPPC monolayer is measured as a function of the surface tension γ of the monolayer. For small enough values of γ, the measured k comes mostly from the bending rigidity K of the DPPC monolayer. This transition occurs at γK ≃ 20 mN/m, which defines a characteristic length λ = (K/γK)^1/2 in the range of nano-meters, given that K ~ 100K_BT.