  Understanding the kinematics and dynamics of spreading, pinching, and
coalescence of drops is critically important for a diverse range of
applications involving spraying, printing, coating, dispensing, emulsification,
and atomization. Hence experimental studies visualize and characterize the
increase in size over time for drops spreading over substrates, or liquid
bridges between coalescing drops, or the decrease in the radius of pinching
necks during drop formation. Even for Newtonian fluids, the interplay of
inertial, viscous, and capillary stresses can lead to a number of scaling laws,
with three limiting self-similar cases: visco-inertial (VI), visco-capillary
(VC) and inertio-capillary (IC). Though experiments are presented as examples
of the methods of dimensional analysis, the lack of precise values or estimates
for pre-factors, transitions, and scaling exponents presents difficulties for
quantitative analysis and material characterization. In this tutorial review,
we reanalyze and summarize an elaborate set of landmark published experimental
studies on a wide range of Newtonian fluids. We show that moving beyond VI, VC,
and IC units in favor of intrinsic timescale and lengthscale determined by all
three material properties (viscosity, surface tension and density), creates a
complementary system that we call the Ohnesorge units. We find that in spite of
large differences in topological features, timescales, and material properties,
the analysis of spreading, pinching and coalescing drops in the Ohnesorge units
results in a remarkable collapse of the experimental datasets, highlighting the
shared and universal features displayed in such flows.
