117 research outputs found
Flowing active liquids in a pipe: Hysteretic response of polar flocks to external fields
We investigate the response of colloidal flocks to external fields. We first
show that individual colloidal rollers align with external flows as would a
classical spin with magnetic fields. Assembling polar active liquids from
colloidal rollers, we experimentally demonstrate their hysteretic response:
confined colloidal flocks can proceed against external flows. We theoretically
explain this collective robustness, using an active hydrodynamic description,
and show how orientational elasticity and confinement protect the direction of
collective motion. Finally, we exploit the intrinsic bistability of confined
active flows to devise self-sustained microfluidic oscillators.Comment: 12 pages, 7 figure; accepted for publication in Physical Review
Elastic interaction between "hard'' or "soft" pointwise inclusions on biological membranes
We calculate the induced elastic-interaction between pointwise membrane
inclusions that locally interact up to quadratic order with the membrane
curvature tensor. For isotropic inclusions, we recover the usual interaction
proportional to the inverse fourth power of the separation, however with a
prefactor showing a non-trivial dependence on the rigidity of the
quadratic potential. In the large limit, corresponding to ``hard''
inclusions, we recover the standard prefactor first obtained by Goulian et al.
[Europhys. Lett. \textbf{22}, 145 (1993)]. In the small limit,
corresponding to "soft" inclusions, we recover the recent result of Marchenko
and Misbah [Eur. Phys. J. E \textbf{8}, 477 (2002)]. This shows that the latter
result bears no fundamental discrepancy with previous works, but simply
corresponds to the limit of soft inclusions. We discuss how the same inclusion
can be depicted as hard or soft according to the degree of coarse-graining of
the pointwise description. Finally, we argue that conical transmembrane
proteins should be fundamentally considered as hard inclusions.Comment: 6 page
Tailoring the interactions between self-propelled bodies
We classify the interactions between self-propelled particles moving at a
constant speed from symmetry considerations. We establish a systematic
expansion for the two-body forces in the spirit of a multipolar expansion. This
formulation makes it possible to rationalize most of the models introduced so
far within a common framework. We distinguish between three classes of physical
interactions: (i) potential forces, (ii) inelastic collisions and (iii)
non-reciprocal interactions involving polar or nematic alignment with an
induced field. This framework provides simple design rules for the modeling and
the fabrication of self-propelled bodies interacting via physical interactions.
A class of possible interactions that should yield new phases of active matter
is highlighted
Effects of intermediate bound states in dynamic force spectroscopy
We revisit here some aspects of the interpretation of dynamic force
spectroscopy experiments. The standard theory predicts a typical unbinding
force linearly proportional to the logarithm of the loading rate when
a single energetical barrier controls the unbinding process; for a more complex
situation of barriers, it predicts at most linear segments for the
vs. curve, each segment characterizing a different barrier. We
here extend this existing picture using a refined approximation, we provide a
more general analytical formula, and show that in principle up to
segments can show up experimentally. As a consequence the interpretation of
data can be ambiguous, for the characteristics and even the number of barriers.
A further possible outcome of a multiple-barrier landscape is a bimodal or
multimodal distribution of the unbinding force at a given loading rate, a
feature recently observed experimentally.Comment: 7 pages, 5 figure
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