riving frequencies and drop sizes, and reported a number of new bouncing states. We
have reported the full bouncing threshold curve and shown that it can be discontinuous.
We observed the existence of two distinct bouncing states corresponding to the same
mode number .m; n/, which we dubbed ‘bouncing’ and ‘vibrating’. The transition
between these two energy levels of the same mode plays a crucial role in the
coalescence of small drops and in the onset of walking for larger drops. For small
drops, the contact time in the .1; 1/1 mode, being roughly half the oscillation period,
is longer than the thinning time of the intervening air layer, resulting in coalescence.
The lower part of the bouncing threshold curve therefore lies along the .1; 1/1–.1; 1/2
transition curve. The peak of the walking region typically occurs on the transition
curve between the .2; 1/1 and the .2; 1/2 modes, as will be shown in MBII. The
dominance of the .2; 1/2 mode beyond this point is responsible for the walking region
having only finite extent.
to the drop’s natural oscillation frequency. For example, when the data describing
the coalescence–bouncing threshold and period-doubling thresholds are described in
terms of this ratio, they collapse onto a single curve. We observe and rationalize the
coexistence of two non-coalescing states, bouncing and vibrating, for identical system
parameters. In the former state, the contact time is prescribed by the drop dynamics;
in the latter, by the driving frequency. The bouncing states are described by theoretical
models of increasing complexity whose predictions are tested against experimental
data. We first model the drop–bath interaction in terms of a linear spring, then develop
a logarithmic spring model that better captures the drop dynamics over a wider range
of parameter space. While the linear spring model provides a faster, less accurate
option, the logarithmic spring model is found to be more accurate and consistent with
all existing data.