  The overall traffic of droplets in a network of microfluidic channels is
strongly influenced by the liquid properties of the moving droplets. In
particular, the effective hydrodynamic resistance of individual droplets plays
a key role in their global behavior. We here propose two simple and low-cost
experimental methods for measuring this parameter by analyzing the dynamics of
a regular sequence of droplets injected into an "asymmetric loop" network. The
choice of a droplet taking either route through the loop is influenced by the
presence of previous droplets which modulate the hydrodynamic resistance of the
branches they are sitting in. We propose to extract the effective resistance of
a droplet from easily observable time series, namely from the choices the
droplets make at junctions and from the inter-droplet distances. This becomes
possible when utilizing a recently proposed theoretical model, based on a
number of simplifying assumptions. We here present several sets of measurements
of the hydrodynamic resistance of droplets, expressed in terms of a "resistance
length". The aim is twofold, (1) to reveal its dependence on a number of
parameters, such as the viscosity, the volume of droplets, their velocity as
well as the spacing between them. At the same time (2), by using a standard
measurement technique, we compare the limitations of the proposed methods. As
an important result of this comparison we obtain the range of validity of the
simplifying assumptions made in the theoretical model.
