1,742 research outputs found
Direct measurement of thermophoretic forces
We study the thermophoretic motion of a micron sized single colloidal
particle in front of a flat wall by evanescent light scattering. To quantify
thermophoretic effects we analyse the nonequilibrium steady state (NESS) of the
particle in a constant temperature gradient perpendicular to the confining
walls. We propose to determine thermophoretic forces from a 'generalized
potential' associated with the probability distribution of the particle
position in the NESS. Experimentally we demonstrate, how this spatial
probability distribution is measured and how thermophoretic forces can be
extracted with 10 fN resolution. By varying temperature gradient and ambient
temperature, the temperature dependence of Soret coefficient is
determined for polystyrene and melamine
particles. The functional form of is in good agreement with findings
for smaller colloids. In addition, we measure and discuss hydrodynamic effects
in the confined geometry. The theoretical and experimental technique proposed
here extends thermophoresis measurements to so far inaccessible particle sizes
and particle solvent combinations
Probing linear and nonlinear microrheology of viscoelastic fluids
Bulk rheological properties of viscoelastic fluids have been extensively
studied in macroscopic shearing geometries. However, little is known when an
active microscopic probe is used to locally perturb them far from the
linear-response regime. Using a colloidal particle dragged periodically by
scanning optical tweezers through a viscoelastic fluid, we investigate both,
its linear and nonlinear microrheological response. With increasing particle
velocity, we observe a transition from constant viscosity to a thinning regime,
where the drag force on the probe becomes a nonlinear function of the particle
velocity. We demonstrate that this transition is only determined by the ratio
of the fluid's equilibrium relaxation time and the period of the driving.Comment: 6 pages, 5 figure
Transient dynamics of a colloidal particle driven through a viscoelastic fluid
We experimentally study the transient motion of a colloidal particle actively
dragged by an optical trap through different viscoelastic fluids (wormlike
micelles, polymer solutions, and entangled -phage DNA). We observe
that, after sudden removal of the moving trap, the particle recoils due to the
recovery of the deformed fluid microstructure. We find that the transient
dynamics of the particle proceeds via a double exponential relaxation, whose
relaxation times remain independent of the initial particle velocity whereas
their amplitudes strongly depend on it. While the fastest relaxation mirrors
the viscous damping of the particle by the solvent, the slow relaxation results
from the recovery of the strained viscoelastic matrix. We show that this
transient information, which has no counterpart in Newtonian fluids, can be
exploited to investigate linear and nonlinear rheological properties of the
embedding fluid, thus providing a novel method to perform transient rheology at
the micron-scale.Comment: 19 pages, 7 Figures, submitted to New Journal of Physic
Run-and-Tumble-Like Motion of Active Colloids in Viscoelastic Media
Run-and-tumble (RNT) motion is a prominent locomotion strategy employed by
many living microorganisms. It is characterized by straight swimming intervals
(runs), which are interrupted by sudden reorientation events (tumbles). In
contrast, directional changes of synthetic microswimmers (active particles,
APs) are caused by rotational diffusion, which is superimposed with their
translational motion and thus leads to rather continuous and slow particle
reorientations. Here we demonstrate that active particles can also perform a
swimming motion where translational and orientational changes are disentangled,
similar to RNT. In our system, such motion is realized by a viscoelastic
solvent and a periodic modulation of the self-propulsion velocity.
Experimentally, this is achieved using light-activated Janus colloids, which
are illuminated by a time-dependent laser field. We observe a strong
enhancement of the effective translational and rotational motion when the
modulation time is comparable to the relaxation time of the viscoelastic fluid.
Our findings are explained by the relaxation of the elastic stress, which
builds up during the self-propulsion, and is suddenly released when the
activity is turned off. In addition to a better understanding of active motion
in viscoelastic surroundings, our results may suggest novel steering strategies
for synthetic microswimmers in complex environments.Comment: 6 figures, New Journal of Physics accepte
Dynamics of self-propelled Janus particles in viscoelastic fluids
We experimentally investigate active motion of spherical Janus colloidal
particles in a viscoelastic fluid. Self-propulsion is achieved by a local
concentration gradient of a critical polymer mixture which is imposed by laser
illumination. Even in the regime where the fluid's viscosity is independent
from the deformation rate induced by the particle, we find a remarkable
increase of up to two orders of magnitude of the rotational diffusion with
increasing particle velocity, which can be phenomenologically described by an
effective rotational diffusion coefficient dependent on the Weissenberg number.
We show that this effect gives rise to a highly anisotropic response of
microswimmers in viscoelastic media to external forces depending on its
orientation.Comment: 5 pages, 4 figures, Physical Review Letters (accepted
Experimental Accessibility of Generalized Fluctuation-Dissipation Relations for Nonequilibrium Steady States
We study the fluctuation-dissipation theorem for a Brownian particle driven
into a nonequilibrium steady state experimentally. We validate two different
theoretical variants of a generalized fluctuation-dissipation theorem.
Furthermore, we demonstrate that the choice of variables crucially affects the
accuracy of determining the nonequilibrium response from steady state
nonequilibrium fluctuations
Measurement of second-order response without perturbation
We study the second order response functions of a colloidal particle being
subjected to an anharmonic potential. Contrary to typical response measurements
which require an external perturbation, here we experimentally confirm a
recently developed approach where the system's susceptibilities up to second
order are obtained from the particle's equilibrium trajectory [PCCP
, 6653 (2015)]. The measured susceptibilities are in
quantitative agreement with those obtained from the response to an external
perturbation.Comment: 4 figure
Phototaxis of synthetic microswimmers in optical landscapes
Many microorganisms, with phytoplankton and zooplankton as prominent
examples, display phototactic behaviour, that is, the ability to perform
directed motion within a light gradient. Here we experimentally demonstrate
that sensing of light gradients can also be achieved in a system of synthetic
photo-activated microparticles being exposed to an inhomogeneous laser field.
We observe a strong orientational response of the particles because of
diffusiophoretic torques, which in combination with an intensity-dependent
particle motility eventually leads to phototaxis. Since the aligning torques
saturate at high gradients, a strongly rectified particle motion is found even
in periodic asymmetric intensity landscapes. Our results are in excellent
agreement with numerical simulations of a minimal model and should similarly
apply to other particle propulsion mechanisms. Because light fields can be
easily adjusted in space and time, this also allows to extend our approach to
dynamical environments.Comment: 10 pages, 7 figure
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