320 research outputs found
Colloidal particles at a nematic-isotropic interface: effects of confinement
When captured by a flat nematic-isotropic interface, colloidal particles can
be dragged by it. As a result spatially periodic structures may appear, with
the period depending on a particle mass, size, and interface
velocity~\cite{west.jl:2002}. If liquid crystal is sandwiched between two
substrates, the interface takes a wedge-like shape, accommodating the
interface-substrate contact angle and minimizing the director distortions on
its nematic side. Correspondingly, particles move along complex trajectories:
they are first captured by the interface and then `glide' towards its vertex
point. Our experiments quantify this scenario, and numerical minimization of
the Landau-de Gennes free energy allow for a qualitative description of the
interfacial structure and the drag force.Comment: 7 pages, 9 figure
Modeling the elastic deformation of polymer crusts formed by sessile droplet evaporation
Evaporating droplets of polymer or colloid solution may produce a glassy
crust at the liquid-vapour interface, which subsequently deforms as an elastic
shell. For sessile droplets, the known radial outward flow of solvent is
expected to generate crusts that are thicker near the pinned contact line than
the apex. Here we investigate, by non-linear quasi-static simulation and
scaling analysis, the deformation mode and stability properties of elastic caps
with a non-uniform thickness profile. By suitably scaling the mean thickness
and the contact angle between crust and substrate, we find data collapse onto a
master curve for both buckling pressure and deformation mode, thus allowing us
to predict when the deformed shape is a dimple, mexican hat, and so on. This
master curve is parameterised by a dimensionless measure of the non-uniformity
of the shell. We also speculate on how overlapping timescales for gelation and
deformation may alter our findings.Comment: 8 pages, 7 figs. Some extra clarification of a few points, and minor
corrections. To appear in Phys. Rev.
Transverse electrokinetic and microfluidic effects in micro-patterned channels: lubrication analysis for slab geometries
Off-diagonal (transverse) effects in micro-patterned geometries are predicted
and analyzed within the general frame of linear response theory, relating
applied presure gradient and electric field to flow and electric current. These
effects could contribute to the design of pumps, mixers or flow detectors.
Shape and charge density modulations are proposed as a means to obtain sizeable
transverse effects, as demonstrated by focusing on simple geometries and using
the lubrication approximation.Comment: 9 pages, 7 figure
Dielectrophoresis of nanocolloids: a molecular dynamics study
Dielectrophoresis (DEP), the motion of polarizable particles in non-uniform
electric fields, has become an important tool for the transport, separation,
and characterization of microparticles in biomedical and nanoelectronics
research. In this article we present, to our knowledge, the first molecular
dynamics simulations of DEP of nanometer-sized colloidal particles. We
introduce a simplified model for polarizable nanoparticles, consisting of a
large charged macroion and oppositely charged microions, in an explicit
solvent. The model is then used to study DEP motion of the particle at
different combinations of temperature and electric field strength. In accord
with linear response theory, the particle drift velocities are shown to be
proportional to the DEP force. Analysis of the colloid DEP mobility shows a
clear time dependence, demonstrating the variation of friction under
non-equilibrium. The time dependence of the mobility further results in an
apparent weak variation of the DEP displacements with temperature
A hysteretic multiscale formulation for validating computational models of heterogeneous structures
A framework for the development of accurate yet computationally efficient numerical models is proposed in this work, within the context of computational model validation. The accelerated computation achieved herein relies on the implementation of a recently derived multiscale finite element formulation, able to alternate between scales of different complexity. In such a scheme, the micro-scale is modelled using a hysteretic finite elements formulation. In the micro-level, nonlinearity is captured via a set of additional hysteretic degrees of freedom compactly described by an appropriate hysteric law, which gravely simplifies the dynamic analysis task. The computational efficiency of the scheme is rooted in the interaction between the micro- and a macro-mesh level, defined through suitable interpolation fields that map the finer mesh displacement field to the coarser mesh displacement field. Furthermore, damage related phenomena that are manifested at the micro-level are accounted for, using a set of additional evolution equations corresponding to the stiffness degradation and strength deterioration of the underlying material. The developed modelling approach is utilized for the purpose of model validation; firstly, in the context of reliability analysis; and secondly, within an inverse problem formulation where the identification of constitutive parameters via availability of acceleration response data is sought
A hysteretic multiscale formulation for nonlinear dynamic analysis of composite materials
This article has been made available through the Brunel Open Access Publishing Fund.A new multiscale finite element formulation
is presented for nonlinear dynamic analysis of heterogeneous
structures. The proposed multiscale approach utilizes
the hysteretic finite element method to model the microstructure.
Using the proposed computational scheme, the micro-basis functions, that are used to map the microdisplacement components to the coarse mesh, are only evaluated once and remain constant throughout the analysis procedure. This is accomplished by treating inelasticity at the micro-elemental level through properly defined hysteretic evolution equations. Two types of imposed boundary conditions are considered for the derivation of the multiscale basis functions, namely the linear and periodic boundary conditions. The validity of the proposed formulation as well as its computational efficiency are verified through illustrative numerical experiments
On the prediction of topology and local properties for optimal trussed structures
A new formulation is presented for mathematical modelling to predict the distribution of material, material properties, and topology for the optimal design of trussed structures. The design problem is cast in a form to minimize a measure of generalized compliance , which is calculated as a sum over the structure of weighted displacement. Member stiffnesses appear as design variables and, starting with a given ground structure, the solution predicts the optimal layout and distribution of stiffness. The isoperimetric constraint in the reformulated problem measures total cost in generalized form , based on independently specified unit relative cost factors for each truss element. One or another form of optimal design is generated via a process where designated elements in the unit relative cost field are adjusted systematically at each cycle. The generalized cost feature provides as well for the introduction of certain technical constraints into the design problem, e.g. the facility to design around obstacles. Results for each cycle of an algorithm for computational treatment are identified as the solution to a properly posed optimization problem. Computational procedures are demonstrated by the prediction of optimal designs for a variety of truss problems in 2D.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46074/1/158_2005_Article_BF01197558.pd
The Role of IL-15 Deficiency in the Pathogenesis of Virus-Induced Asthma Exacerbations
Rhinovirus infections are the major cause of asthma exacerbations. We hypothesised that IL-15, a cytokine implicated in innate and acquired antiviral immunity, may be deficient in asthma and important in the pathogenesis of asthma exacerbations. We investigated regulation of IL-15 induction by rhinovirus in human macrophages in vitro, IL-15 levels in bronchoalveolar lavage (BAL) fluid and IL-15 induction by rhinovirus in BAL macrophages from asthmatic and control subjects, and related these to outcomes of infection in vivo. Rhinovirus induced IL-15 in macrophages was replication-, NF-κB- and α/β interferon-dependent. BAL macrophage IL-15 induction by rhinovirus was impaired in asthmatics and inversely related to lower respiratory symptom severity during experimental rhinovirus infection. IL-15 levels in BAL fluid were also decreased in asthmatics and inversely related with airway hyperresponsiveness and with virus load during in vivo rhinovirus infection. Deficient IL-15 production in asthma may be important in the pathogenesis of asthma exacerbations
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