85 research outputs found
Isotropic-Cholesteric Transition of a Weakly Chiral Elastomer Cylinder
When a chiral isotropic elastomer is brought to low temperature cholesteric
phase, the nematic degree of freedom tends to order and form a helix. Due to
the nemato-elastic coupling, this also leads to elastic deformation of the
polymer network that is locally coaxial with the nematic order. However, the
helical structure of nematic order is incompatible with the energetically
preferred elastic deformation. The system is therefore frustrated and
appropriate compromise has to be achieved between the nematic ordering and the
elastic deformation. For a strongly chiral elastomer whose pitch is much
smaller than the system size, this problem has been studied by Pelcotivs and
Meyer, as well as by Warner. In this work, we study the isotropic-cholesteric
transition in the weak chirality limit, where the pitch is comparable or much
larger than system size. We compare two possible solutions: a helical state as
well as a double twist state. We find that the double twist state very
efficiently minimizes both the elastic free energy and the chiral nematic free
energy. On the other hand, the pitch of the helical state is strongly affected
by the nemato-elastic coupling. As a result this state is not efficient in
minimizing the chiral nematic free energy.Comment: 7 pages, 2 eps figure
Nonreciprocity as a generic route to traveling states
We examine a non-reciprocally coupled dynamical model of a mixture of two
diffusing species. We demonstrate that nonreciprocity, which is encoded in the
model via antagonistic cross diffusivities, provides a generic mechanism for
the emergence of traveling patterns in purely diffusive systems with
conservative dynamics. In the absence of non-reciprocity, the binary fluid
mixture undergoes a phase transition from a homogeneous mixed state to a
demixed state with spatially separated regions rich in one of the two
components. Above a critical value of the parameter tuning non-reciprocity, the
static demixed pattern acquires a finite velocity, resulting in a state that
breaks both spatial and time translational symmetry, as well as the reflection
parity of the static pattern. We elucidate the generic nature of the transition
to traveling patterns using a minimal model that can be studied analytically.
Our work has direct relevance to nonequilibrium assembly in mixtures of
chemically interacting colloids that are known to exhibit non-reciprocal
effective interactions, as well as to mixtures of active and passive agents
where traveling states of the type predicted here have been observed in
simulations. It also provides insight on transitions to traveling and
oscillatory states seen in a broad range of nonreciprocal systems with
non-conservative dynamics, from reaction-diffusion and prey-predators models to
multispecies mixtures of microorganisms with antagonistic interactions.Comment: 8 pages, 3 figure
Re-enterant efficiency of phototaxis in Chlamydomonas reinhardtii cells
Phototaxis is one of the most fundamental stimulus-response behaviors in
biology wherein motile micro-organisms sense light gradients to swim towards
the light source. Apart from single cell survival and growth, it plays a major
role at the global scale of aquatic ecosystem and bio-reactors. We study
photoaxis of single celled algae Chalmydomonas reinhardtii as a function of
cell number density and light stimulus using high spatio-temporal video
microscopy. Surprisingly, the phototactic efficiency has a minimum at a
well-defined number density, for a given light gradient, above which the
phototaxis behaviour of collection of cells can even exceed the performance
obtainable from single isolated cells. We show that the origin of enhancement
of performance above the critical concentration lies in the slowing down of the
cells which enables them to sense light more effectively. We also show that
this steady state phenomenology is well captured by a modelling the phototactic
response as a density dependent torque acting on an active Brownian particle
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