  We study the effects of strain on the properties and dynamics of Wannier
excitons in monolayer (phosphorene) and few-layer black phosphorus (BP), a
promising two-dimensional material for optoelectronic applications due to its
high mobility, mechanical strength and strain-tuneable direct band gap. We
compare the results to the case of molybdenum disulphide (MoS$_2$) monolayers.
We find that the so-called funnel effect, i.e. the possibility of controlling
exciton motion by means of inhomogeneous strains, is much stronger in few-layer
BP than in MoS$_2$ monolayers and, crucially, is of opposite sign. Instead of
excitons accumulating isotropically around regions of high tensile strain like
in MoS$_2$, excitons in BP are pushed away from said regions. This
\emph{inverse} funnel effect is moreover highly anisotropic, with much larger
funnel distances along the armchair crystallographic direction, leading to a
directional focusing of exciton flow. A strong inverse funnel effect could
enable simpler designs of funnel solar cells, and offer new possibilities for
the manipulation and harvesting of light.
