  We present the results of a search for unknown interactions that couple to
mass between an optically levitated microsphere and a gold-coated silicon
cantilever. The scale and geometry of the apparatus enables a search for new
forces that appear at distances below 100 $\mu$m and which would have evaded
previous searches due to screening mechanisms. The data are consistent with
electrostatic backgrounds and place upper limits on the strength of new
interactions at $<0.1$ fN in the geometry tested. For the specific example of a
chameleon interaction with an inverse power law potential, these results
exclude matter couplings $\beta > 5.6 \times 10^4$ in the region of parameter
space where the self-coupling $\Lambda \gtrsim 5$ meV and the microspheres are
not fully screened.

High Energy Physics - Experiment;Cosmology and Nongalactic Astrophysics;Optics