  Recent experimental realizations of the critical Casimir effect have been
implemented by monitoring colloidal particles immersed in a binary liquid
mixture near demixing and exposed to a chemically structured substrate. In
particular, critical Casimir forces have been measured for surfaces consisting
of stripes with periodically alternating adsorption preferences, forming
chemical steps between them. Motivated by these experiments, we analyze the
contribution of such chemical steps to the critical Casimir force for the film
geometry and within the Ising universality class. By means of Monte Carlo
simulations, mean-field theory, and finite-size scaling analysis we determine
the universal scaling function associated with the contribution to the critical
Casimir force due to individual, isolated chemical steps facing a surface with
homogeneous adsorption preference or with Dirichlet boundary condition. In line
with previous findings, these results allow one to compute the critical Casimir
force for the film geometry and in the presence of arbitrarily shaped, but wide
stripes. In this latter limit the force decomposes into a sum of the
contributions due to the two homogeneous parts of the surface and due to the
chemical steps between the stripes. We assess this decomposition by comparing
the resulting sum with actual simulation data for the critical Casimir force in
the presence of a chemically striped substrate.
