137 research outputs found
Characterizing Potentials by a Generalized Boltzmann Factor
Based on the concept of a nonequilibrium steady state, we present a novel
method to experimentally determine energy landscapes acting on colloidal
systems. By measuring the stationary probability distribution and the current
in the system, we explore potential landscapes with barriers up to several
hundred \kT. As an illustration, we use this approach to measure the
effective diffusion coefficient of a colloidal particle moving in a tilted
potential
Noninvasive Measurement of Dissipation in Colloidal Systems
According to Harada and Sasa [Phys. Rev. Lett. 95, 130602 (2005)], heat
production generated in a non-equilibrium steady state can be inferred from
measuring response and correlation functions. In many colloidal systems,
however, it is a nontrivial task to determine response functions, whereas
details about spatial steady state trajectories are easily accessible. Using a
simple conditional averaging procedure, we show how this fact can be exploited
to reliably evaluate average heat production. We test this method using
Brownian dynamics simulations, and apply it to experimental data of an
interacting driven colloidal system
The Einstein relation generalized to non-equilibrium
The Einstein relation connecting the diffusion constant and the mobility is
violated beyond the linear response regime. For a colloidal particle driven
along a periodic potential imposed by laser traps, we test the recent
theoretical generalization of the Einstein relation to the non-equilibrium
regime which involves an integral over measurable velocity correlation
functions
Thermodynamics of a Colloidal Particle in a Time-Dependent Non-Harmonic Potential
We study the motion of an overdamped colloidal particle in a time-dependent
non-harmonic potential. We demonstrate the first law-like balance between
applied work, exchanged heat, and internal energy on the level of a single
trajectory. The observed distribution of applied work is distinctly
non-Gaussian in good agreement with numerical calculations. Both the Jarzynski
relation and a detailed fluctuation theorem are verified with good accuracy
Finite sampling effects on generalized fluctuation-dissipation relations for steady states
We study the effects of the finite number of experimental data on the
computation of a generalized fluctuation-dissipation relation around a
nonequilibrium steady state of a Brownian particle in a toroidal optical trap.
We show that the finite sampling has two different effects, which can give rise
to a poor estimate of the linear response function. The first concerns the
accessibility of the generalized fluctuation-dissipation relation due to the
finite number of actual perturbations imposed to the control parameter. The
second concerns the propagation of the error made at the initial sampling of
the external perturbation of the system. This can be highly enhanced by
introducing an estimator which corrects the error of the initial sampled
condition. When these two effects are taken into account in the data analysis,
the generalized fluctuation-dissipation relation is verified experimentally
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
Free energy of colloidal particles at the surface of sessile drops
The influence of finite system size on the free energy of a spherical
particle floating at the surface of a sessile droplet is studied both
analytically and numerically. In the special case that the contact angle at the
substrate equals a capillary analogue of the method of images is
applied in order to calculate small deformations of the droplet shape if an
external force is applied to the particle. The type of boundary conditions for
the droplet shape at the substrate determines the sign of the capillary
monopole associated with the image particle. Therefore, the free energy of the
particle, which is proportional to the interaction energy of the original
particle with its image, can be of either sign, too. The analytic solutions,
given by the Green's function of the capillary equation, are constructed such
that the condition of the forces acting on the droplet being balanced and of
the volume constraint are fulfilled. Besides the known phenomena of attraction
of a particle to a free contact line and repulsion from a pinned one, we
observe a local free energy minimum for the particle being located at the drop
apex or at an intermediate angle, respectively. This peculiarity can be traced
back to a non-monotonic behavior of the Green's function, which reflects the
interplay between the deformations of the droplet shape and the volume
constraint.Comment: 24 pages, 19 figure
Entropy production for mechanically or chemically driven biomolecules
Entropy production along a single stochastic trajectory of a biomolecule is
discussed for two different sources of non-equilibrium. For a molecule
manipulated mechanically by an AFM or an optical tweezer, entropy production
(or annihilation) occurs in the molecular conformation proper or in the
surrounding medium. Within a Langevin dynamics, a unique identification of
these two contributions is possible. The total entropy change obeys an integral
fluctuation theorem and a class of further exact relations, which we prove for
arbitrarily coupled slow degrees of freedom including hydrodynamic
interactions. These theoretical results can therefore also be applied to driven
colloidal systems. For transitions between different internal conformations of
a biomolecule involving unbalanced chemical reactions, we provide a
thermodynamically consistent formulation and identify again the two sources of
entropy production, which obey similar exact relations. We clarify the
particular role degenerate states have in such a description
Career-Success Scale – A new instrument to assess young physicians' academic career steps
Background: Within the framework of a prospective cohort study of Swiss medical school graduates, a Career-Success Scale (CSS) was constructed in a sample of young physicians choosing different career paths in medicine. Furthermore the influence of personality factors, the participants' personal situation, and career related factors on their career success was investigated.
Methods: 406 residents were assessed in terms of career aspired to, and their career progress. The Career-Success Scale, consisting of 7 items, was developed and validated, addressing objective criteria of academic career advancement. The influence of gender and career aspiration was investigated by a two-factorial analysis of variance, the relationships between personality factors, personal situation, career related factors and the Career-Success Scale by a multivariate linear regression analysis.
Results: The unidimensional Career-Success Scale has an internal consistency of 0.76. It is significantly correlated at the bivariate level with gender, instrumentality, and all career related factors, particularly with academic career and received mentoring. In multiple regression, only gender, academic career, surgery as chosen specialty, and received mentoring are significant predictors. The highest values were observed in participants aspiring to an academic career, followed by those pursuing a hospital career and those wanting to run a private practice. Independent of the career aspired to, female residents have lower scores than their male colleagues.
Conclusion: The Career-Success Scale proved to be a short, reliable and valid instrument to measure career achievements. As mentoring is an independent predictor of career success, mentoring programs could be an important instrument to specifically enhance careers of female
physicians in academia
Single-molecule experiments in biological physics: methods and applications
I review single-molecule experiments (SME) in biological physics. Recent
technological developments have provided the tools to design and build
scientific instruments of high enough sensitivity and precision to manipulate
and visualize individual molecules and measure microscopic forces. Using SME it
is possible to: manipulate molecules one at a time and measure distributions
describing molecular properties; characterize the kinetics of biomolecular
reactions and; detect molecular intermediates. SME provide the additional
information about thermodynamics and kinetics of biomolecular processes. This
complements information obtained in traditional bulk assays. In SME it is also
possible to measure small energies and detect large Brownian deviations in
biomolecular reactions, thereby offering new methods and systems to scrutinize
the basic foundations of statistical mechanics. This review is written at a
very introductory level emphasizing the importance of SME to scientists
interested in knowing the common playground of ideas and the interdisciplinary
topics accessible by these techniques. The review discusses SME from an
experimental perspective, first exposing the most common experimental
methodologies and later presenting various molecular systems where such
techniques have been applied. I briefly discuss experimental techniques such as
atomic-force microscopy (AFM), laser optical tweezers (LOT), magnetic tweezers
(MT), biomembrane force probe (BFP) and single-molecule fluorescence (SMF). I
then present several applications of SME to the study of nucleic acids (DNA,
RNA and DNA condensation), proteins (protein-protein interactions, protein
folding and molecular motors). Finally, I discuss applications of SME to the
study of the nonequilibrium thermodynamics of small systems and the
experimental verification of fluctuation theorems. I conclude with a discussion
of open questions and future perspectives.Comment: Latex, 60 pages, 12 figures, Topical Review for J. Phys. C (Cond.
Matt
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