New dissipated energy for nonnegative weak solution of unstable thin-film equations

Abstract

The fluid thin film equation $h_t = - (h^n h_{xxx})_x - a_1\,(h^m h_x)_x$ is known to conserve mass $\int\,h \, dx$, and in the case of $a_1 \leq 0$, to dissipate entropy $\int\,h^{3/2 - n}\,dx$ (see [8]) and the $L^2$-norm of the gradient $\int\,h_x^2\,dx$ (see [3]). For the special case of $a_1 = 0$ a new dissipated quantity $\int\, h^{\alpha}\,h_x^2\,dx $ was recently discovered for positive classical solutions by Laugesen (see [15]). We extend it in two ways. First, we prove that Laugesen's functional dissipates strong nonnegative generalized solutions. Second, we prove the full $\alpha$-energy $\int\,\bigl(\frac{1}{2} \,h^\alpha \, h_x^2\, - \frac{a_1\,h^{\alpha + m - n + 2}}{(\alpha + m - n + 1)(\alpha + m - n + 2)} \bigr)\, dx $ dissipation for strong nonnegative generalized solutions in the case of the unstable porous media perturbation $a_1> 0$ and the critical exponent $m = n+2$.