On formation of dry spots in heated liquid films

Here, the phenomenon of food sticking when frying in a frying pan is experimentally explained. Thermocapillary convection causes a dry spot formation in the center of the frying pan upon heating of the sunflower oil film. It is shown that the speed of formation of a dry spot is similar to the speed of receding motion of the edge of a droplet upon impact and spreading on a solid surface. This allows theoretical determination of the speed of dewetting. For the thin liquid film flowing vertically over a solid surface, the critical volumetric flow rate q_cr partitions two regimes: metastable or subcritical, when small perturbation of the film free surface results in the film rupture (q < q_cr) and stable or supercritical at q > q_cr. For the falling thin liquid film, the critical volumetric flow rate q_cr partitions two regimes: metastable or subcritical (q < q_cr) and stable or supercritical at q > q_cr. At q < q_cr, small deformations of the film free surface result in the film rupture. For the case of the temperature distribution in the form of a unit step function, the fundamental solution G₁(x) describing the deformation of the film free surface has been derived by the perturbation technique. This solution is important by itself since it describes the most “dangerous” film surface profile at a prescribed value of the temperature drop. For an arbitrary surface temperature distribution θ (ξ), the convolution of G₁(ξ) and θ ′(ξ) yields the film thickness profile.