49 research outputs found
Extrapolation to nonequilibrium from coarse grained response theory
Nonlinear response theory, in contrast to linear cases, involves (dynamical)
details, and this makes application to many body systems challenging. From the
microscopic starting point we obtain an exact response theory for a small
number of coarse grained degrees of freedom. With it, an extrapolation scheme
uses near-equilibrium measurements to predict far from equilibrium properties
(here, second order responses). Because it does not involve system details,
this approach can be applied to many body systems. It is illustrated in a four
state model and in the near critical Ising model.Comment: Accepted for publication in Phys. Rev. Let
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
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
Force measurement in the presence of Brownian noise: Equilibrium distribution method vs. Drift method
The study of microsystems and the development of nanotechnologies require new
techniques to measure piconewton and femtonewton forces at microscopic and
nanoscopic scales. Amongst the challenges, there is the need to deal with the
ineluctable thermal noise, which, in the typical experimental situation of a
spatial diffusion gradient, causes a spurious drift. This leads to a correction
term when forces are estimated from drift measurements [Phys. Rev. Lett. 104,
170602 (2010)]. Here, we provide a systematic study of such effect comparing
the forces acting on various Brownian particles derived from equilibrium
distribution and drift measurements. We discuss the physical origin of the
correction term, its dependence on wall distance, particle radius, and its
relation to the convention used to solve the respective stochastic integrals.
Such correction term becomes more significant for smaller particles and is
predicted to be in the order of several piconewtons for particles the size of a
biomolecule.Comment: 10 pages, 13 figure
Critical Casimir forces in colloidal suspensions on chemically patterned surfaces
We investigate the behavior of colloidal particles immersed in a binary
liquid mixture of water and 2,6-lutidine in the presence of a chemically
patterned substrate. Close to the critical point of the mixture, the particles
are subjected to critical Casimir interactions with force components normal and
parallel to the surface. Because the strength and sign of these interactions
can be tuned by variations in the surface properties and the mixtures
temperature, critical Casimir forces allow the formation of highly ordered
monolayers but also extend the use of colloids as model systems.Comment: 4 papges, 4 figures, accepted at Phys. Rev. Let
Influence of Noise on Force Measurements
We demonstrate how the ineluctable presence of thermal noise alters the
measurement of forces acting on microscopic and nanoscopic objects. We quantify
this effect exemplarily for a Brownian particle near a wall subjected to
gravitational and electrostatic forces. Our results demonstrate that the force
measurement process is prone to artifacts if the noise is not correctly taken
into account.Comment: 4 Pages, 4 Figures, Accepte
Novel perspectives for the application of total internal reflection microscopy
Total Internal Reflection Microscopy (TIRM) is a sensitive non-invasive
technique to measure the interaction potentials between a colloidal particle
and a wall with femtonewton resolution. The equilibrium distribution of the
particle-wall separation distance z is sampled monitoring the intensity I
scattered by the Brownian particle under evanescent illumination. Central to
the data analysis is the knowledge of the relation between I and the
corresponding z, which typically must be known a priori. This poses
considerable constraints to the experimental conditions where TIRM can be
applied (short penetration depth of the evanescent wave, transparent surfaces).
Here, we introduce a method to experimentally determine I(z) by relying only on
the distance-dependent particle-wall hydrodynamic interactions. We demonstrate
that this method largely extends the range of conditions accessible with TIRM,
and even allows measurements on highly reflecting gold surfaces where multiple
reflections lead to a complex I(z).Comment: 11 pages, 7 figure
Salt-induced changes of colloidal interactions in critical mixtures
We report on salt-dependent interaction potentials of a single charged
particle suspended in a binary liquid mixture above a charged wall. For
symmetric boundary conditions (BC) we observe attractive particle-wall
interaction forces which are similar to critical Casimir forces previously
observed in salt-free mixtures. However, in case of antisymmetric BC we find a
temperature-dependent crossover from attractive to repulsive forces which is in
strong contrast to salt-free conditions. Additionally performed small-angle
x-ray scattering experiments demonstrate that the bulk critical fluctuations
are not affected by the addition of salt. This suggests that the observed
crossover can not be attributed alone to critical Casimir forces. Instead our
experiments point towards a possible coupling between the ionic distributions
and the concentration profiles in the binary mixture which then affects the
interaction potentials in such systems.Comment: 5 pages, 4 Figure
Terminal NK cell maturation is controlled by concerted actions of T-bet and Zeb2 and is essential for melanoma rejection
Natural killer (NK) cell maturation is a tightly controlled process that endows NK cells with functional competence and the capacity to recognize target cells. Here, we found that the transcription factor (TF) Zeb2 was the most highly induced TF during NK cell maturation. Zeb2 is known to control epithelial to mesenchymal transition, but its role in immune cells is mostly undefined. Targeted deletion of Zeb2 resulted in impaired NK cell maturation, survival, and exit from the bone marrow. NK cell function was preserved, but mice lacking Zeb2 in NK cells were more susceptible to B16 melanoma lung metastases. Reciprocally, ectopic expression of Zeb2 resulted in a higher frequency of mature NK cells in all organs. Moreover, the immature phenotype of Zeb2(-/-) NK cells closely resembled that of Tbx21(-/-) NK cells. This was caused by both a dependence of Zeb2 expression on T-bet and a probable cooperation of these factors in gene regulation. Transgenic expression of Zeb2 in Tbx21(-/-) NK cells partially restored a normal maturation, establishing that timely induction of Zeb2 by T-bet is an essential event during NK cell differentiation. Finally, this novel transcriptional cascade could also operate in human as T-bet and Zeb2 are similarly regulated in mouse and human NK cells