Thermal effects on liquid film dynamics in spin coating

We numerically and theoretically study thermoviscosity (temperature dependent viscosity) and thermocapillarity (temperature dependent surface tension) effects on thin liquid film dynamics in the spin coating process when the rotating disk is heated axisymmetrically in the radial direction. A nonlinear scalar equation for the film thickness profile evolution is derived under lubrication assumptions. Our numerical results show that both thermocapillarity and thermoviscosity effects can be harnessed to enhance the liquid films depletion during spin coating when the disk center is at a higher temperature than the disk outer edge. Thermoviscosity effect is important and dominates the external air shearing effect when the film thickness is relatively large. Thermoviscosity is shown to introduce unevenness to the spin coated film but the magnitude of unevenness decreases when the film thickness is reduced. When the liquid film is thin enough, thermocapillarity effect starts to dominate centrifugal force, external air shearing and thermoviscosity effects. A theoretical solution for the film thickness evolution is obtained when the thermoviscosity effect is negligible. When the applied disk temperature profile has a steep change, a double shock structure for the liquid film is generated. The new shock wave formation phenomenon is induced by a sudden viscosity change due to a sudden temperature change.