561 research outputs found
Pattern formation in colloidal explosions
We study the non-equilibrium pattern formation that emerges when magnetically
repelling colloids, trapped by optical tweezers, are abruptly released, forming
colloidal explosions. For multiple colloids in a single trap we observe a
pattern of expanding concentric rings. For colloids individually trapped in a
line, we observe explosions with a zigzag pattern that persists even when
magnetic interactions are much weaker than those that break the linear symmetry
in equilibrium. Theory and computer simulations quantitatively describe these
phenomena both in and out of equilibrium. An analysis of the mode spectrum
allows us to accurately quantify the non-harmonic nature of the optical traps.
Colloidal explosions provide a new way to generate well-characterized
non-equilibrium behaviour in colloidal systems.Comment: New restructured version (supplementary material goes into main text,
no change of content), added journal reference and DOI information; 6 pages,
6 figures, published in Europhysics Letters (EPL
Criticality and phase separation in a two-dimensional binary colloidal fluid induced by the solvent critical behavior
We present an experimental and theoretical study of the phase behavior of a
binary mixture of colloids with opposite adsorption preferences in a critical
solvent. As a result of the attractive and repulsive critical Casimir forces,
the critical fluctuations of the solvent lead to a further critical point in
the colloidal system, i.e. to a critical colloidal-liquid--colloidal-liquid
demixing phase transition which is controlled by the solvent temperature. Our
experimental findings are in good agreement with calculations based on a simple
approximation for the free energy of the system.Comment: 5 pages, 5 figures, to be published in Europhysics Letter
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
Tunability of Critical Casimir Interactions by Boundary Conditions
We experimentally demonstrate that critical Casimir forces in colloidal
systems can be continuously tuned by the choice of boundary conditions. The
interaction potential of a colloidal particle in a mixture of water and
2,6-lutidine has been measured above a substrate with a gradient in its
preferential adsorption properties for the mixture's components. We find that
the interaction potentials at constant temperature but different positions
relative to the gradient continuously change from attraction to repulsion. This
demonstrates that critical Casimir forces respond not only to minute
temperature changes but also to small changes in the surface properties.Comment: 4 figures;
http://www.iop.org/EJ/article/0295-5075/88/2/26001/epl_88_2_26001.htm
Hysteresis and re-entrant melting of a self-organized system of classical particles confined in a parabolic trap
A self-organized system composed of classical particles confined in a
two-dimensional parabolic trap and interacting through a potential with a
short-range attractive part and long-range repulsive part is studied as
function of temperature. The influence of the competition between the
short-range attractive part of the inter-particle potential and its long-range
repulsive part on the melting temperature is studied. Different behaviors of
the melting temperature are found depending on the screening length ()
and the strength () of the attractive part of the inter-particle potential.
A re-entrant behavior and a thermal induced phase transition is observed in a
small region of ()-space. A structural hysteresis effect is observed
as a function of temperature and physically understood as due to the presence
of a potential barrier between different configurations of the system.Comment: 8 pages, 6 figure
Theory of orientational ordering in colloidal molecular crystals
Freezing of charged colloids on square or triangular two-dimensional periodic
substrates has been recently shown to realize a rich variety of orientational
orders. We propose a theoretical framework to analyze the corresponding
structures. A fundamental ingredient is that a non spherical charged object in
an electrolyte creates a screened electrostatic potential that is anisotropic
at any distance. Our approach is in excellent agreement with the known
experimental and numerical results, and explains in simple terms the reentrant
orientational melting observed in these so called colloidal molecular crystals.
We also investigate the case of a rectangular periodic substrate and predict an
unusual phase transition between orientationnaly ordered states, as the aspect
ratio of the unit cell is changed.Comment: 4 pages, to appear in Phys. Rev. Let
Universal symmetry of optimal control at the microscale
Optimizing the energy efficiency of driving processes provides valuable
insights into the underlying physics and is of crucial importance for numerous
applications, from biological processes to the design of machines and robots.
Knowledge of optimal driving protocols is particularly valuable at the
microscale, where energy supply is often limited. Here we investigate
experimentally and theoretically the paradigmatic optimization problem of
moving a potential carrying a load through a fluid, in a finite time and over a
given distance, in such a way that the required work is minimal. An important
step towards more realistic systems is the consideration of memory effects in
the surrounding fluid, which are ubiquitous in real-world applications.
Therefore, our experiments were performed in viscous and viscoelastic media,
which are typical environments for synthetic and biological processes on the
microscale. Despite marked differences between the protocols in both fluids, we
find that the optimal control protocol and the corresponding average particle
trajectory always obey a time-reversal symmetry. We show that this symmetry,
which surprisingly applies here to a class of processes far from thermal
equilibrium, holds universally for various systems, including active, granular,
and long-range correlated media in their linear regimes. The uncovered symmetry
provides a rigorous and versatile criterion for optimal control that greatly
facilitates the search for energy-efficient transport strategies in a wide
range of systems. Using a machine learning algorithm, we demonstrate that the
algorithmic exploitation of time-reversal symmetry can significantly enhance
the performance of numerical optimization algorithms.Comment: 16 pages with 10 figures, accepted for publication in PR
Experimental observation of the Aubry transition in two-dimensional colloidal monolayers
The possibility to achieve entirely frictionless, i.e. superlubric, sliding
between solids, holds enormous potential for the operation of mechanical
devices. At small length scales, where mechanical contacts are well-defined,
Aubry predicted a transition from a superlubric to a pinned state when the
mechanical load is increased. Evidence for this intriguing Aubry transition
(AT), which should occur in one dimension (1D) and at zero temperature, was
recently obtained in few-atom chains. Here, we experimentally and theoretically
demonstrate the occurrence of the AT in an extended two-dimensional (2D) system
at room temperature using a colloidal monolayer on an optical lattice. Unlike
the continuous nature of the AT in 1D, we observe a first-order transition in
2D leading to a coexistence regime of pinned and unpinned areas. Our data
demonstrate that the original concept of Aubry does not only survive in 2D but
is relevant for the design of nanoscopic machines and devices at ambient
temperature.Comment: 12 pages including 4 figures + 9 pages supplemental informatio
Measuring the equation of state of a hard-disc fluid
We use video microscopy to study a two-dimensional (2D) model fluid of
charged colloidal particles suspended in water and compute the pressure from
the measured particle configurations. Direct experimental control over the
particle density by means of optical tweezers allows the precise measurement of
pressure as a function of density. We compare our data with theoretical
predictions for the equation of state, the pair-correlation function and the
compressibility of a hard-disc fluid and find good agreement, both for the
fluid and the solid phase. In particular the location of the transition point
agrees well with results from Monte Carlo simulations.Comment: 7 pages, to appear in EPL, slightly corrected versio
Biophysical Investigations Elucidating the Mechanisms of Action of Antimicrobial Peptides and Their Synergism
Biophysical and structural investigations are presented with a focus on the membrane lipid interactions of cationic linear antibiotic peptides such as magainin, PGLa, LL37, and melittin. Observations made with these peptides are distinct as seen from data obtained with the hydrophobic peptide alamethicin. The cationic amphipathic peptides predominantly adopt membrane alignments parallel to the bilayer surface; thus the distribution of polar and non-polar side chains of the amphipathic helices mirror the environmental changes at the membrane interface. Such a membrane partitioning of an amphipathic helix has been shown to cause considerable disruptions in the lipid packing arrangements, transient openings at low peptide concentration, and membrane disintegration at higher peptide-to-lipid ratios. The manifold supramolecular arrangements adopted by lipids and peptides are represented by the 'soft membranes adapt and respond, also transiently' (SMART) model. Whereas molecular dynamics simulations provide atomistic views on lipid membranes in the presence of antimicrobial peptides, the biophysical investigations reveal interesting details on a molecular and supramolecular level, and recent microscopic imaging experiments delineate interesting sequences of events when bacterial cells are exposed to such peptides. Finally, biophysical studies that aim to reveal the mechanisms of synergistic interactions of magainin 2 and PGLa are presented, including unpublished isothermal titration calorimetry (ITC), circular dichroism (CD) and dynamic light scattering (DLS) measurements that suggest that the peptides are involved in liposome agglutination by mediating intermembrane interactions. A number of structural events are presented in schematic models that relate to the antimicrobial and synergistic mechanism of amphipathic peptides when they are aligned parallel to the membrane surface.PMC602300
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