  We present an immersed boundary method to simulate the creeping motion of a
rigid particle in a fluid described by the Stokes equations discretized thanks
to a finite element strategy on unfitted meshes, called Phi-FEM, that uses the
description of the solid with a level-set function. One of the advantages of
our method is the use of standard finite element spaces and classical
integration tools, while maintaining the optimal convergence (theoretically in
the H1 norm for the velocity and L2 for pressure; numerically also in the L2
norm for the velocity).
