Thermocapillary deformation of thin liquid films by infrared irradiation

  • H.M.J.M. Wedershoven

Student thesis: Master


The goal of this study was to investigate the suitability of temperature gradients induced by an infrared laser beam for controlling the morphology and break-up of a thin liquid film that is deposited on a solid substrate. We first measured the intensity profile of the infrared laser beam with a scanning slit, at different positions along the propagation direction of the beam. Deviations from a Gaussian intensity profile were attributed to the spherical aberration of the optical system. The absorption of the laser power by the substrate results in a non-uniform temperature distribution. We developed an axisymmetric numerical model to obtain this time dependent temperature profile. The results from this model were compared to the results from an analytical model, based on the method of Green's functions. We systematically studied the effects of the laser power and the beam diameter on the temperature profile. Both models were also extended to the case where the substrate is moving relative to the laser beam. The heat transfer model for a stationary substrate was experimentally verified with thermocouple measurements. A thin film of a non-volatile liquid was spin coated on the solid substrate. The non-uniform temperature distribution of the substrate results in a deformation of the thin film due to thermocapillary stress. We systematically measured the deformation of the thin film using dual-wavelength interferometry. We developed a numerical model for the deformation, based on the lubrication approximation. We studied the effect of the initial film thickness and the laser power in the case of a stationary substrate and additionally the effect of the substrate speed in the case of a moving substrate. We achieved good agreement between the measurements and numerical simulations, once the temperature dependence of the viscosity was taken into account. In the case of a partially wetting substrate the thin film becomes unstable, ruptures and dewets the substrate. We measured the time at which film rupture occurs as a function of the laser power and achieved good agreement with numerical simulations. The numerical model used a phenomenological expression for the disjoining pressure.
Date of Award30 Sep 2013
Original languageEnglish
SupervisorC.W.J. Berendsen (Supervisor 1), Jos C.H. Zeegers (Supervisor 2) & Anton A. Darhuber (Supervisor 2)

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