  Small water droplets or particles located at an oil meniscus typically climb
the meniscus due to unbalanced capillary forces. Here, we introduce a
size-dependent reversal of this meniscus-climbing behavior, where upon cooling
of the underlying substrate, droplets of different sizes concurrently ascend
and descend the meniscus. We show that microscopic Marangoni convection cells
within the oil meniscus are responsible for this phenomenon. While dynamics of
relatively larger water microdroplets are still dominated by unbalanced
capillary forces and hence ascend the meniscus, smaller droplets are carried by
the surface flow and consequently descend the meniscus. We further demonstrate
that the magnitude and direction of the convection cells depend on the meniscus
geometry and the substrate temperature and introduce a modified Marangoni
number that well predicts their strength. Our findings provide a new approach
to manipulating droplets on a liquid meniscus that could have applications in
material self-assembly, biological sensitive sensing and testing, or phase
change heat transfer.
