140 research outputs found
Capillary focusing close to a topographic step: Shape and instability of confined liquid filaments
Step-emulsification is a microfluidic technique for droplet generation which
relies on the abrupt decrease of confinement of a liquid filament surrounded by
a continuous phase. A striking feature of this geometry is the transition
between two distinct droplet breakup regimes, the "step-regime" and
"jet-regime", at a critical capillary number. In the step-regime, small and
monodisperse droplets break off from the filament directly at a topographic
step, while in the jet-regime a jet protrudes into the larger channel region
and large plug-like droplets are produced. We characterize the breakup behavior
as a function of the filament geometry and the capillary number and present
experimental results on the shape and evolution of the filament for a wide
range of capillary numbers in the jet-regime. We compare the experimental
results with numerical simulations. Assumptions based on the smallness of the
depth of the microfluidic channel allow to reduce the governing equations to
the Hele-Shaw problem with surface tension. The full nonlinear equations are
then solved numerically using a volume-of-fluid based algorithm. The
computational framework also captures the transition between both regimes,
offering a deeper understanding of the underlying breakup mechanism
Granular Response to Impact: Topology of the Force Networks
Impact of an intruder on granular matter leads to formation of mesoscopic
force networks seen particularly clearly in the recent experiments carried out
with photoelastic particles, e.g., Clark et al., Phys. Rev. Lett., 114 144502
(2015). These force networks are characterized by complex structure and evolve
on fast time scales. While it is known that total photoelastic activity in the
granular system is correlated with the acceleration of the intruder, it is not
known how the structure of the force network evolves during impact, and if
there is a dominant features in the networks that can be used to describe
intruder's dynamics. Here, we use topological tools, in particular persistent
homology, to describe these features. Persistent homology allows quantification
of both structure and time evolution of the resulting force networks. We find
that there is a clear correlation of the intruder's dynamics and some of the
topological measures implemented. This finding allows us to discuss which
properties of the force networks are most important when attempting to describe
intruder's dynamics. Regarding temporal evolution of the networks, we are able
to define the upper bound on the relevant time scale on which the networks
evolve
Thin films flowing down inverted substrates: Three dimensional flow
We study contact line induced instabilities for a thin film of fluid under
destabilizing gravitational force in three dimensional setting. In the previous
work (Phys. Fluids, {\bf 22}, 052105 (2010)), we considered two dimensional
flow, finding formation of surface waves whose properties within the
implemented long wave model depend on a single parameter,
, where is the capillary number and is
the inclination angle. In the present work we consider fully 3D setting and
discuss the influence of the additional dimension on stability properties of
the flow. In particular, we concentrate on the coupling between the surface
instability and the transverse (fingering) instabilities of the film front. We
furthermore consider these instabilities in the setting where fluid viscosity
varies in the transverse direction. It is found that the flow pattern strongly
depends on the inclination angle and the viscosity gradient
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