  Hele-Shaw problems are prototypes to study the interface dynamics. Linear
theory suggests the existence of self-similar patterns in a Hele-Shaw flow.
That is, with a specific injection flux the interface shape remains unchanged
while its size increases. In this paper, we explore the existence of
self-similar patterns in the nonlinear regime and develop a rigorous nonlinear
theory characterizing their fundamental features. Using a boundary integral
formulation, we pose the question of self-similarity as a generalized nonlinear
eigenvalue problem, involving two nonlinear integral operators. The flux
constant $C$ is the eigenvalue and the corresponding self-similar pattern
$\mathbf{x}$ is the eigenvector. We develop a quasi-Newton method to solve the
problem and show the existence of nonlinear shapes with $k$-fold dominated
symmetries. The influence of initial guesses on the self-similar patterns is
investigated. We are able to obtain a desired self-similar shape once the
initial guess is properly chosen. Our results go beyond the predictions of
linear theory and establish a bridge between the linear theory and simulations.
