31 research outputs found
Linear stability of an active fluid interface
Motivated by studies suggesting that the patterns exhibited by the
collectively expanding fronts of thin cells during the closing of a wound [Mark
et al., Biophys. J., 98:361-370, 2010] and the shapes of single cells crawling
on surfaces [Callan-Jones et al., Phys. Rev. Lett., 100:258106, 2008] are due
to fingering instabilities, we investigate the stability of actively driven
interfaces under Hele-Shaw confinement. An initially radial interface between a
pair of viscous fluids is driven by active agents. Surface tension and bending
rigidity resist deformation of the interface. A point source at the origin and
a distributed source are also included to model the effects of injection or
suction, and growth or depletion, respectively. Linear stability analysis
reveals that for any given initial radius of the interface, there are two key
dimensionless driving rates that determine interfacial stability. We discuss
stability regimes in a state space of these parameters and their implications
for biological systems. An interesting finding is that an actively mobile
interface is susceptible to fingering instability irrespective of viscosity
contrast
Coarse-graining intramolecular hydrodynamic interaction in dilute solutions of flexible polymers
We present a scheme for coarse-graining hydrodynamic interactions in an isolated flexible homopolymer
molecule in solution. In contrast to the conventional bead-spring model that employs spherical beads of fixed
radii to represent the hydrodynamic characteristics of coarse-grained segments, we show that our procedure
leads naturally to a discrete model of a polymer molecule as a chain of orientable and stretchable Gaussian
blobs. This model accounts for both intrablob and interblob hydrodynamic interactions, which depend on the
instantaneous shapes of the blobs. In Brownian dynamics simulations of initially stretched chains relaxing
under quiescent conditions, the transient evolution of the mean-square end-to-end distance and first normal
stress difference obtained with the Gaussian-blob model are found to be less sensitive to the degree of coarse
graining, in comparison with the conventional bead-spring model with Rotne-Prager-Yamakawa hydrodynamic
interactions
Extensional viscosity of copper nanowire suspensions in an aqueous polymer solution
Suspensions of copper nanowires are emerging as new electronic inks for
next-generation flexible electronics. Using a novel surface acoustic wave
driven extensional flow technique we are able to perform currently lacking
analysis of these suspensions and their complex buffer. We observe extensional
viscosities from 3 mPas (1 mPas shear viscosity) to 37.2
Pas via changes in the suspension concentration, thus capturing low
viscosities that have been historically very challenging to measure. These
changes equate to an increase in the relative extensional viscosity of nearly
12,200 times at a volume fraction of just 0.027. We also find that interactions
between the wires and the necessary polymer additive affect the rheology
strongly. Polymer-induced elasticity shows a reduction as the buffer relaxation
time falls from 819 to 59 s above a critical particle concentration. The
results and technique presented here should aid in the future formulation of
these promising nanowire suspensions and their efficient application as inks
and coatings.Comment: 7 pages, 5 figures, under review for Soft Matter RS
Motility induced changes in viscosity of suspensions of swimming microbes in extensional flows
Suspensions of motile cells are model systems for understanding the unique
mechanical properties of living materials which often consist of ensembles of
self-propelled particles. We present here a quantitative comparison of theory
against experiment for the rheology of such suspensions. The influence of
motility on viscosities of cell suspensions is studied using a novel
acoustically-driven microfluidic capillary-breakup extensional rheometer.
Motility increases the extensional viscosity of suspensions of algal pullers,
but decreases it in the case of bacterial or sperm pushers. A recent model
[Saintillan, Phys. Rev. E, 2010, 81:56307] for dilute active suspensions is
extended to obtain predictions for higher concentrations, after independently
obtaining parameters such as swimming speeds and diffusivities. We show that
details of body and flagellar shape can significantly determine macroscale
rheological behaviour.Comment: 12 pages, 1 appendix, 7 figures, submitted to Soft Matter - under
revie