Dynamics and linear stability of thermocapillary spreading films on homogeneous and micropatterned surfaces.

摘要

The recent focus on microfluidic devices has generated substantial interest in small-scale transport phenomena. Because the surface to volume ratio scales inversely with the characteristic length scale, surface forces dominate in microscale systems. In particular, these forces can be manipulated to regulate the motion of thin liquid films. The dynamics and stability of thermocapillary spreading films are theoretically investigated in this dissertation for flow on homogeneous and chemically or topographically patterned substrates. Because the governing equations for spreading films driven by other forces are analogous, the approach and results are valid for general lubrication flows. Experiments have shown that films spreading on homogeneous substrates can undergo a flow transition from a uniform front at the advancing solid-liquid-vapor contact line to an array of parallel rivulets. This instability is investigated via a non-modal, transient analysis because the relevant linearized disturbance operators for spatially inhomogeneous thin films are nonnormal. Stability results for three different contact line models are compared.; This investigation of thermocapillary driven spreading is also pursued in the context of characterizing a novel, open-architecture microfluidic device based on flow confinement to completely wetting microstripes through chemical micropatterning of the substrate. The resulting lateral curvature of the fluid significantly influences the dynamics of the liquid. Applied to the dip coating of these patterned substrates, hydrodynamic scaling arguments are used to derive a replacement for the classical Landau-Levich result for homogeneous substrates. Thermocapillary flow along wetting microstripes is then characterized. The lateral curvature modifies the expected spreading velocity and film profile and also suppresses the capillary ridge and instability observed at the advancing contact line on homogeneous surfaces. In addition, a lubrication-based model is derived to quantify the significant effects of lateral film curvature and fluid confinement on the transverse diffusive broadening in two microstreams merging at a $⋎$-junction. Finally, the analysis is extended to lubrication flow over chemically uniform but topographically patterned substrates. A transient analysis is employed to determine the evolution of disturbances to the capillary ridges induced by the substrate topography.