527 research outputs found
Drag on particles in a nematic suspension by a moving nematic-isotropic interface
We report the first clear demonstration of drag on colloidal particles by a moving nematic-isotropic
interface. The balance of forces explains our observation of periodic, strip-like structures that are produced by the movement of these particles
Undulation instabilities in the meniscus of smectic membranes
Using optical microscopy, phase shifting interferometry and atomic force
microscopy, we demonstrate the existence of undulated structures in the
meniscus of ferroelectric smectic-C* films. The meniscus is characterized by a
periodic undulation of the smectic-air interface, which manifests itself in a
striped pattern. The instability disappears in the untilted smectic-A phase.
The modulation amplitude and wavelength both depend on meniscus thickness. We
study the temperature evolution of the structure and propose a simple model
that accounts for the observed undulations.Comment: Submitted to PR
Azimuthal instability of the radial thermocapillary flow around a hot bead trapped at the water-air interface
We investigate the radial thermocapillary flow driven by a laser-heated
microbead in partial wetting at the water-air interface. Particular attention
is paid to the evolution of the convective flow patterns surrounding the hot
sphere as the latter is increasingly heated. The flow morphology is nearly
axisymmetric at low laser power P. Increasing P leads to symmetry breaking with
the onset of counter-rotating vortex pairs. The boundary condition at the
interface, close to no-slip in the low-P regime, turns about stress-free
between the vortex pairs in the high-P regime. These observations strongly
support the view that surface-active impurities are inevitably adsorbed on the
water surface where they form an elastic layer. The onset of vortex pairs is
the signature of a hydrodynamic instability in the layer response to the
centrifugal forced flow. Interestingly, our study paves the way for the design
of active colloids able to achieve high-speed self-propulsion via vortex pair
generation at a liquid interface
Orientational and phase-coexistence behaviour of hard rod-sphere mixtures
Results are presented from Monte Carlo simulations of bulk mixtures of Hard Gaussian Overlap particles with an aspect ratio of 3:1 and hard spheres with diameters equal to the breadths of the rods. For sphere number-concentrations of 50% and lower, compression of the isotropic fluid results in formation of a homogeneous (i.e. compositionally mixed) nematic phase. The volume fraction of this isotropic-nematic transition is found to increase approximately linearly with sphere concentration. On compression to higher volume fractions, however, this homogeneous nematic phase separates out into coexisting nematic and isotropic phases.</p
Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality and Defects
We calculate the interaction between two spherical colloidal particles
embedded in the isotropic phase of a nematogenic liquid. The surface of the
particles induces wetting nematic coronas that mediate an elastic interaction.
In the weak wetting regime, we obtain exact results for the interaction energy
and the texture, showing that defects and biaxiality arise, although they are
not topologically required. We evidence rich behaviors, including the
possibility of reversible colloidal aggregation and dispersion. Complex
anisotropic self-assembled phases might be formed in dense suspensions.Comment: 4 pages, 6 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
Identification of QTLs controlling gene expression networks defined a priori
BACKGROUND: Gene expression microarrays allow the quantification of transcript accumulation for many or all genes in a genome. This technology has been utilized for a range of investigations, from assessments of gene regulation in response to genetic or environmental fluctuation to global expression QTL (eQTL) analyses of natural variation. Current analysis techniques facilitate the statistical querying of individual genes to evaluate the significance of a change in response, also known as differential expression. Since genes are also known to respond as groups due to their membership in networks, effective approaches are needed to investigate transcriptome variation as related to gene network responses. RESULTS: We describe a statistical approach that is capable of assessing higher-order a priori defined gene network response, as measured by microarrays. This analysis detected significant network variation between two Arabidopsis thaliana accessions, Bay-0 and Shahdara. By extending this approach, we were able to identify eQTLs controlling network responses for 18 out of 20 a priori-defined gene networks in a recombinant inbred line population derived from accessions Bay-0 and Shahdara. CONCLUSION: This approach has the potential to be expanded to facilitate direct tests of the relationship between phenotypic trait and transcript genetic architecture. The use of a priori definitions for network eQTL identification has enormous potential for providing direction toward future eQTL analyses
Simulating Particle Dispersions in Nematic Liquid-Crystal Solvents
A new method is presented for mesoscopic simulations of particle dispersions
in nematic liquid crystal solvents. It allows efficient first-principle
simulations of the dispersions involving many particles with many-body
interactions mediated by the solvents. A simple demonstration is shown for the
aggregation process of a two dimentional dispersion.Comment: 5 pages, 5 figure
Direct measurements of the effects of salt and surfactant on interaction forces between colloidal particles at water-oil interfaces
The forces between colloidal particles at a decane-water interface, in the
presence of low concentrations of a monovalent salt (NaCl) and of the
surfactant sodium dodecylsulfate (SDS) in the aqueous subphase, have been
studied using laser tweezers. In the absence of electrolyte and surfactant,
particle interactions exhibit a long-range repulsion, yet the variation of the
interaction for different particle pairs is found to be considerable. Averaging
over several particle pairs was hence found to be necessary to obtain reliable
assessment of the effects of salt and surfactant. It has previously been
suggested that the repulsion is consistent with electrostatic interactions
between a small number of dissociated charges in the oil phase, leading to a
decay with distance to the power -4 and an absence of any effect of electrolyte
concentration. However, the present work demonstrates that increasing the
electrolyte concentration does yield, on average, a reduction of the magnitude
of the interaction force with electrolyte concentration. This implies that
charges on the water side also contribute significantly to the electrostatic
interactions. An increase in the concentration of SDS leads to a similar
decrease of the interaction force. Moreover the repulsion at fixed SDS
concentrations decreases over longer times. Finally, measurements of three-body
interactions provide insight into the anisotropic nature of the interactions.
The unique time-dependent and anisotropic interactions between particles at the
oil-water interface allow tailoring of the aggregation kinetics and structure
of the suspension structure.Comment: Submitted to Langmui
Collective dynamics of colloids at fluid interfaces
The evolution of an initially prepared distribution of micron sized colloidal
particles, trapped at a fluid interface and under the action of their mutual
capillary attraction, is analyzed by using Brownian dynamics simulations. At a
separation \lambda\ given by the capillary length of typically 1 mm, the
distance dependence of this attraction exhibits a crossover from a logarithmic
decay, formally analogous to two-dimensional gravity, to an exponential decay.
We discuss in detail the adaption of a particle-mesh algorithm, as used in
cosmological simulations to study structure formation due to gravitational
collapse, to the present colloidal problem. These simulations confirm the
predictions, as far as available, of a mean-field theory developed previously
for this problem. The evolution is monitored by quantitative characteristics
which are particularly sensitive to the formation of highly inhomogeneous
structures. Upon increasing \lambda\ the dynamics show a smooth transition from
the spinodal decomposition expected for a simple fluid with short-ranged
attraction to the self-gravitational collapse scenario.Comment: 13 pages, 12 figures, revised, matches version accepted for
publication in the European Physical Journal
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