Existence and uniqueness for several non-linear elliptic problems arising in lubrication theory

We consider the non-linear elliptic equation ∇·A(x,p)∇p=∇·b(x,p) with positive Dirichlet boundary data. The coefficients A and b are taken from models used in lubrication theory, and in particular are not defined for negative values of p. We prove some general existence and uniqueness results for a family of models, which extend related results in the literature. These results allow us to prove existence, uniqueness and positivity of the solution to advanced compressible lubrication models such as the kinetic-based Fukui-Kaneko model and the second-order-slip model. We also consider a spring-like model of compliant-foil compressible bearing, and weaken some hypotheses of previous results on more classical models such as the standard Reynolds model and the first-order-slip model.