Existence and Uniqueness for Double-Phase Poisson Equations with Variable Growth

We study a class of nonlinear elliptic problems driven by a double-phase operator with variable exponents, arising in the modeling of heterogeneous materials undergoing phase transitions. The associated Poisson problem features a combination of two distinct growth conditions, modulated by a measurable weight function \( μ\), leading to spatially varying ellipticity. Working within the framework of modular function spaces, we establish the uniform convexity of the modular associated with the gradient term. This structural property enables a purely variational treatment of the problem. As a consequence, we prove existence and uniqueness of weak solutions under natural and minimal assumptions on the variable exponents and the weight.