J. H. Hwang

Bio: J. H. Hwang is an academic researcher. The author has contributed to research in topics: Surface roughness & Thin film. The author has an hindex of 1, co-authored 1 publications receiving 53 citations.

On the flow of a thin liquid film over a rough rotating disk

Abstract: This study addresses the depletion of thin liquid films due to centrifugation. Equations describing the flow of a thin viscous film over a rough rotating disk are solved by either an analytical or numerical method. Depletion histories of the film are given for cases involving regular patterns of surface asperities. It is found that surface roughness markedly enhances the retention of a thin liquid film on a rotating disk. Moreover, different topographic structures of the surface roughness lead to different asymptotic limits of liquid retention.

Impact of drops on solid surfaces : self-similar capillary waves, and splashing as a new type of kinematic discontinuity

Abstract: The impact of drops impinging one by one on a solid surface is studied experimentally and theoretically. The impact process is observed by means of a charge-coupled-device camera, its pictures processed by computer. Low-velocity impact results in spreading and in propagation of capillary waves, whereas at higher velocities splashing (i.e. the emergence of a cloud of small secondary droplets, absent in the former case) sets in. Capillary waves are studied in some detail in separate experiments. The dynamics of the extension of liquid lamellae produced by an impact in the case of splashing is recorded. The secondary-droplet size distributions and the total volume of these droplets are measured, and the splashing threshold is found as a function of the impact parameters.The pattern of the capillary waves is predicted to be self-similar. The calculated wave profile agrees well with the experimental data. It is shown theoretically that the splashing threshold corresponds to the onset of a velocity discontinuity propagating over the liquid layer on the wall. This discontinuity shows several aspects of a shock. In an incompressible liquid such a discontinuity can only exist in the presence of a sink at its front. The latter results in the emergence of a circular crown-like sheet virtually normal to the wall and propagating with the discontinuity. It is predicted theoretically and recorded in the experiment. The crown is unstable owing to the formation of cusps at the free rim at its top edge, which results in the splashing effect. The onset velocity of splashing and the rate of propagation of the kinematic discontinuity are calculated and the theoretical results agree fairly well with the experimental data. The structure of the discontinuity is shown to match the outer solution.

Experimental study of substrate roughness and surfactant effects on the Landau-Levich law

Abstract: In this work we present an experimental study of deviations from the classical Landau-Levich law in the problem of dip coating. Among the examined causes leading to deviations are the nature of the liquid-gas and liquid-solid interfaces. The thickness of the coating film created by withdrawal of a plate from a bath was measured gravimetrically over a wide range of capillary numbers for both smooth and well-characterized rough substrates, and for clean and surfactant interface cases. In view of the dependence of the lifetime of a film on the type of liquid and substrate, and liquid-gas and liquid-solid interfaces, we characterized the range of measurability of the film thickness in the parameter space defined by the withdrawal capillary number, the surfactant concentration, and substrate roughness size. We then study experimentally the effect of a film thickening due to the presence of surfactants. Our recent theory based on a purely hydrodynamic role of the surface active substance suggests that there is a sorption-controlled coating regime in which Marangoni effects should lead to film thinning. However, our experiments conducted in this regime demonstrate film thickening, calling into question the conventional wisdom, which is that Marangoni stresses (as accounted by the conventional interfacial boundary conditions) lead to film thickening. Next we examine the effect of well-characterized substrate roughness on the coated film thickness, which also reveals its influence on wetting-related processes and an effective boundary condition at the wall. In particular, it is found that roughness results in a significant thickening of the film relative to that on a smooth substrate and a different power of capillary number than the classical Landau-Levich law.

Spin coating over topography

Abstract: A model for predicting film thickness profiles around topographical features during spin coating is presented. This model is applicable to features of arbitrary geometry in the two lateral dimensions. This generally permits study of the planarization of real device structures, including both isolated and neighboring features, with any orientation with respect to the wafer center. Predictions from this model agree qualitatively with measured thin-film profiles from interferograms taken during spinning. Phenomena such as pile-up and wakes result from interactions between surface tension and other driving forces in the flow. >