Nonlinear mechanics of soft fibrous networks

Mechanical networks of fibres arise on a range of scales in nature and technology, from the
cytoskeleton of a cell to blood clots, from textiles and felts to skin and collageneous tissues.
Their collective response is dependent on the individual response of the constituent filaments
as well as density, topology and order in the network. Here, we use the example of a lowdensity synthetic felt of athermal filaments to study the generic features of the mechanical
response of such networks including strain stiffening and large effective Poisson ratios.
A simple microscopic model allows us to explain these features of our observations, and
provides us with a baseline framework to understand active biomechanical networks.

Sensorimotor control during isothermal tracking in Caenorhabditis elegans

In order to purposefully navigate their environments,
animals rely on precise coordination between their sensory
and motor systems. The integrated performance of circuits
for sensorimotor control may be analyzed by quantifying
an animal’s motile behavior in defined sensory
environments. Here, we analyze the ability of the
nematode C. elegans to crawl isothermally in spatial
thermal gradients by quantifying the trajectories of
individual worms responding to defined spatiotemporal
thermal gradients. We show that sensorimotor control
during isothermal tracking may be summarized as a
strategy in which the worm changes the curvature of
its propulsive undulations in response to temperature
changes measured at its head. We show that a concise
mathematical model for this strategy for sensorimotor
control is consistent with the exquisite stability of the
worm’s isothermal alignment in spatial thermal gradients
as well as its more complex trajectories in spatiotemporal
thermal gradients.

Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcements

Cytoskeletal microtubules have been proposed
to infl uence cell shape and mechanics based
on their ability to resist large-scale compressive
forces exerted by the surrounding contractile cytoskeleton.
Consistent with this, cytoplasmic microtubules are often
highly curved and appear buckled because of compressive loads. However, the results of in vitro studies suggest that microtubules should buckle at much larger length
scales, withstanding only exceedingly small compressive
forces. This discrepancy calls into question the structural
role of microtubules, and highlights our lack of quantitative
knowledge of the magnitude of the forces they experience
and can withstand in living cells. We show that intracellular microtubules do bear large-scale compressive loads
from a variety of physiological forces, but their buckling
wavelength is reduced signifi cantly because of mechanical coupling to the surrounding elastic cytoskeleton. We
quantitatively explain this behavior, and show that this
coupling dramatically increases the compressive forces
that microtubules can sustain, suggesting they can make a
more signifi cant structural contribution to the mechanical
behavior of the cell than previously thought possible

Crack-front instability in a confined elastic film

We study the undulatory instability of a straight crack front generated by peeling a
flexible elastic plate from a thin elastomeric adhesive film. We show that there is a
threshold for the onset of the instability that is dependent on the ratio of two lengthscales that arise naturally in the problem: the thickness of the film and an elastic length
defined by the stiffness of the plate and that of the film. A linear stability analysis
predicts that the wavelength of the instability scales linearly with the film thickness. Our
results are qualitatively and quantitatively consistent with recent experiments, and show
how crack fronts may lose stability due to a competition between bulk and surface effects
in the presence of multiple length scales.

Transitions to nematic states in homogeneous suspensions of high aspect ratio magnetic rods

Isotropic-nematic and nematic-nematic transitions from a homogeneous suspension of high aspect
ratio magnetic rods are studied for both Maier-Saupe and Onsager excluded volume potentials.
Asymptotic analysis in the vicinity of critical points yields insight into the stability and type of
polarized nematic states emanating from nonpolarized equilibrium states. This, in conjunction with
recently published global numerical results, yields a unified picture of the bifurcation diagram and
provides a convenient base state to study effects of external orienting fields.