Abstract
We call thick those films for which the disjoining pressure and thermal fluctuations are ineffective. Water films with thickness $h$ in the $1-100~\unicodeSTIXx03BCm$ range are thick, but are also known, paradoxically, to nucleate holes spontaneously. We have uncovered a mechanism solving the paradox, relying on the extreme sensitivity of the film to surface tension inhomogeneities. The surface tension of a free liquid film is lowered by an amount $\unicodeSTIXx0394\unicodeSTIXx1D70E$ over a size $a$ by chemical or thermal contamination. At the same time this spot diffuses (within a time $a^2/D$, with $D$ the diffusion coefficient of the pollutant in the substrate), the Marangoni stress $\unicodeSTIXx0394\unicodeSTIXx1D70E/a$ induces an inhomogeneous outward interstitial flow which digs the film within a time $\unicodeSTIXx1D70F_0\unicodeSTIXx1D70Cha^2/\unicodeSTIXx0394\unicodeSTIXx1D70E$, with $\unicodeSTIXx1D70C$ the density of the liquid. When the Péclet number $Pe=a^2/D\unicodeSTIXx1D70F_0$ is larger than unity, the liquid substrate motion reinforces the surface tension gradient, triggering a self-sustained instability insensitive to diffusional regularisation. Several experimental illustrations of the phenomenon are given, both qualitative and quantitative, including a precise study of the first instants of the unstable dynamics made by controlled perturbations of a Savart sheet at large $Pe$.