arXiv subjects

Peter D. Olmsted

Publications and source records attributed to Peter D. Olmsted.

At least 37 records · Page 2Linked to original sources

arXiv2017

Deformation of an Amorphous Polymer during the Fused-Filament-Fabrication Method for Additive Manufacturing

3D printing is rapidly becoming an effective means of prototyping and creating custom consumer goods. The most common method for printing a polymer melt is fused filament fabrication (FFF), and involves extrusion of a thermoplastic material through a heated nozzle; the material is then built up layer-by-layer to fabricate a three-dimensional object. Under typical printing conditions the melt experiences high strain rates within the FFF nozzle, which are able to significantly stretch and orient the polymer molecules. In this paper, we model the deformation of an amorphous polymer melt during the extrusion process, where the fluid must make a 90$^\text{o}$ turn. The melt is described by a modified version of the Rolie-Poly model, which allows for flow-induced changes in the entanglement density. The complex polymer configurations in the cross-section of a printed layer are quantified and visualised. The deposition process involving the corner flow geometry dominates the deformation and significantly disentangles the melt.

cond-mat.soft↗

arXiv2016

Comment on "Elastic Membrane Deformations Govern Interleaflet Coupling of Lipid-Ordered Domains"

In lieu of abstract, first paragraph reads: Galimzyanov et al. [1] find that line tension between thick liquid-ordered ($L_{o}$) and thinner liquid-disordered ($L_{d}$) registered lipid bilayer phases is minimised by an asymmetric "slip region", length $L\!\sim\!5\,\textrm{nm}$ (Fig. 1). They claim that line tensions alone explain domain registration, without "direct" (area-dependent) inter-leaflet interaction [2,3]. We show this is unfounded, without direct interaction their results would predict \textit{antiregistration}, dependent on composition. To find equilibrium from line energies, line \textit{tensions} must be combined with interfacial lengths for given states at given composition. This was not done in [1].

cond-mat.soft↗

arXiv2016

The Physics of Stratum Corneum Lipid Membranes

The Stratum Corneum (SC), the outermost layer of skin, comprises rigid corneocytes (keratin filled dead cells) in a specialized lipid matrix. The continuous lipid matrix provides the main barrier against uncontrolled water loss and invasion of external pathogens. Unlike all other biological lipid membranes (like intracellular organelles and plasma membranes), molecules in SC lipid matrix show small hydrophilic group and large variability in the length of the alkyl tails and in the numbers and positions of groups that are capable of forming hydrogen bonds. Molecular simulations provide a route for systematically probing the effects of each of these differences separately. In this article we present results from atomistic molecular dynamics of selected lipid bilayers and multilayers to probe the effect of these polydispersities. We address the nature of the tail packing in the gel-like phase, the hydrogen bond network among head groups, the bending moduli expected for leaflets comprising SC lipids, and the conformation of very long ceramide lipids (EOS) in multibilayer lipid assemblies.

cond-mat.soft↗

arXiv2015

Kinetics of symmetry and asymmetry in a phase-separating bilayer membrane

We simulate a phase-separating a bilayer in which the leaflets experience a direct coupling favouring local compositional symmetry ("registered" bilayer phases), and an indirect coupling due to hydrophobic mismatch that favours strong local asymmetry ("antiregistered" bilayer phases). For wide ranges of overall leaflet compositions, multiple competing states are possible. For estimated physical parameters, a quenched bilayer may first evolve toward a metastable state more asymmetric than if the leaflets were uncorrelated;\ subsequently, it must nucleate to reach its equilibrium, more symmetric, state. These phase-transition kinetics exhibit characteristic signatures through which fundamental and opposing inter-leaflet interactions may be probed. We emphasise how bilayer phase diagrams with a separate axis for each leaflet can account for overall and local symmetry/asymmetry, and capture a range of observations in the experiment and simulation literature.

cond-mat.soft↗

arXiv2015

Nucleation of symmetric domains in the coupled leaflets of a bilayer

We study the kinetics governing the attainment of inter-leaflet domain symmetry in a phase-separating amphiphilic bilayer. "Indirect" inter-leaflet coupling via hydrophobic mismatch can induce an instability towards a metastable pattern of locally asymmetric domains upon quenching from high temperature. This necessitates a nucleation step to form the conventional symmetric pattern of domains, which are favoured by a "direct" inter-leaflet coupling. We model the energetics for a symmetric domain to nucleate from the metastable state, and find that an interplay between hydrophobic mismatch and thickness stretching/compression causes the effective hydrophobic mismatch, and thus line tension, to depend on domain size. This leads to strong departure from classical nucleation theory. We speculate on implications for cell membrane rafts or clusters, whose size may be of similar magnitude to estimated critical radii for domain symmetry.

cond-mat.soft↗

arXiv2015

Registered and antiregistered phase separation of mixed amphiphilic bilayers

We derive a mean-field free energy for the phase behaviour of coupled bilayer leaflets, which is implicated in cellular processes and important to the design of artificial membranes. Our model accounts for amphiphile-level structural features, particularly hydrophobic mismatch, which promotes antiregistration (AR), in competition with the `direct' trans-midplane coupling usually studied, promoting registration (R). We show that the phase diagram of coupled leaflets allows multiple \textit{metastable} coexistences, then illustrate the kinetic implications with a detailed study of a bilayer of equimolar overall composition. For approximate parameters estimated to apply to phospholipids, equilibrium coexistence is typically registered, but metastable antiregistered phases can be kinetically favoured by hydrophobic mismatch. Thus a bilayer in the spinodal region can require nucleation to equilibrate, in a novel manifestation of Ostwald's `rule of stages'. Our results provide a framework for understanding disparate existing observations, elucidating a subtle competition of couplings, and a key role for phase transition kinetics in bilayer phase behaviour.

cond-mat.soft↗

arXiv2014

Adsorption at Liquid Interfaces Induces Amyloid Fibril Bending and Ring Formation

Protein fibril accumulation at interfaces is an important step in many physiological processes and neurodegenerative diseases as well as in designing materials. Here we show, using $β$-lactoglobulin fibrils as a model, that semiflexible fibrils exposed to a surface do not possess the Gaussian distribution of curvatures characteristic for wormlike chains, but instead exhibit a spontaneous curvature, which can even lead to ring-like conformations. The long-lived presence of such rings is confirmed by atomic force microscopy, cryogenic scanning electron microscopy and passive probe particle tracking at air- and oil-water interfaces. We reason that this spontaneous curvature is governed by structural characteristics on the molecular level and is to be expected when a chiral and polar fibril is placed in an inhomogeneous environment such as an interface. By testing $β$-lactoglobulin fibrils with varying average thicknesses, we conclude that fibril thickness plays a determining role in the propensity to form rings.

q-bio.BM↗

arXiv2014

Fast cholesterol flip-flop and lack of swelling in skin lipid multilayers

Atomistic simulations were performed on hydrated model lipid multilayers that are representative of the lipid matrix in the outer skin (stratum corneum). We find that cholesterol transfers easily between adjacent leaflets belonging to the same bilayer via fast orientational diffusion (tumbling) in the inter-leaflet disordered region, while at the same time there is a large free energy cost against swelling. This fast flip-flop may play an important role in accommodating the variety of curvatures that would be required in the three dimensional arrangement of the lipid multilayers in skin, and for enabling mechanical or hydration induced strains without large curvature elastic costs.

cond-mat.soft↗

arXiv2014

Lamellar and inverse micellar structures of skin lipids: Effect of templating

The outermost layer of skin, the stratum corneum (SC), comprises rigid corneocytes in a layered lipid matrix. Using atomistic simulations we find that the equilibrium phase of the SC lipids is inverse micellar. A model of the corneocyte is used to demonstrate that lamellar layering is induced by the patterned corneocyte wall. The inverse micellar phase is consistent with in vivo observations in the lacunar spaces and at the stratum granulosum - SC boundary region, and suggests a functional role in the lipid synthesis pathway in vivo.

cond-mat.soft↗

arXiv2013

Apparent Fracture in Polymeric Fluids under Step Shear

Recent step strain experiments in well-entangled polymeric liquids demonstrated a bulk fracture-like phenomenon. We have studied this instability using a modern version of the Doi-Edwards theory for entangled polymers, and we find close quantitative agreement with the experiments. The phenomenon occurs because the viscoelastic liquid is sheared into a rubbery state that possesses an elastic constitutive instability (Marrucci and Grizzuti, 1983). The fracture is a transient manifestation of this instability, which relies on the amplification of spatially inhomogeneous fluctuations. This mechanism differs from fracture in glassy materials and dense suspensions.

physics.flu-dyn↗

arXiv2011

Loss of solutions in shear banding fluids in shear banding fluids driven by second normal stress differences

Edge fracture occurs frequently in non-Newtonian fluids. A similar instability has often been reported at the free surface of fluids undergoing shear banding, and leads to expulsion of the sample. In this paper the distortion of the free surface of such a shear banding fluid is calculated by balancing the surface tension against the second normal stresses induced in the two shear bands, and simultaneously requiring a continuous and smooth meniscus. We show that wormlike micelles typically retain meniscus integrity when shear banding, but in some cases can lose integrity for a range of average applied shear rates during which one expects shear banding. This meniscus fracture would lead to ejection of the sample as the shear banding region is swept through. We further show that entangled polymer solutions are expected to display a propensity for fracture, because of their much larger second normal stresses. These calculations are consistent with available data in the literature. We also estimate the meniscus distortion of a three band configuration, as has been observed in some wormlike micellar solutions in a cone and plate geometry.

cond-mat.soft↗

arXiv2010

Nano-scale mechanical probing of supported lipid bilayers with atomic force microscopy

We present theory and experiments for the force-distance curve $F(z_0)$ of an atomic force microscope (AFM) tip (radius $R$) indenting a supported fluid bilayer (thickness $2d$). For realistic conditions the force is dominated by the area compressibility modulus $κ_A$ of the bilayer, and, to an excellent approximation, given by $F= πκ_A R z_0^2/(2d-z_0)^2$. The experimental AFM force curves from coexisting liquid ordered and liquid disordered domains in 3-component lipid bilayers are well-described by our model, and provides $κ_A$ in agreement with literature values. The liquid ordered phase has a yield like response that we model by hydrogen bond breaking.

physics.bio-ph↗

arXiv2010

Undulation instability in a bilayer lipid membrane due to electric field interaction with lipid dipoles

Bilayer lipid membranes [BLMs] are an essential component of all biological systems, forming a functional barrier for cells and organelles from the surrounding environment. The lipid molecules that form membranes contain both permanent and induced dipoles, and an electric field can induce the formation of pores when the transverse field is sufficiently strong (electroporation). Here, a phenomenological free energy is constructed to model the response of a BLM to a transverse static electric field. The model contains a continuum description of the membrane dipoles and a coupling between the headgroup dipoles and the membrane tilt. The membrane is found to become unstable through buckling modes, which are weakly coupled to thickness fluctuations in the membrane. The thickness fluctuations, along with the increase in interfacial area produced by membrane buckling, increase the probability of localized membrane breakdown, which may lead to pore formation. The instability is found to depend strongly on the strength of the coupling between the dipolar headgroups and the membrane tilt as well as the degree of dipolar ordering in the membrane.

physics.bio-ph↗

arXiv2009

Coarse-grained simulations of flow-induced nucleation in semi-crystalline polymers

We perform kinetic Monte Carlo simulations of flow-induced nucleation in polymer melts with an algorithm that is tractable even at low undercooling. The configuration of the non-crystallized chains under flow is computed with a recent non-linear tube model. Our simulations predict both enhanced nucleation and the growth of shish-like elongated nuclei for sufficiently fast flows. The simulations predict several experimental phenomena and theoretically justify a previously empirical result for the flow-enhanced nucleation rate. The simulations are highly pertinent to both the fundamental understanding and process modeling of flow-induced crystallization in polymer melts.

cond-mat.soft↗

arXiv2009

Water permeation through stratum corneum lipid bilayers from atomistic simulations

Stratum corneum, the outermost layer of skin, consists of keratin filled rigid non-viable corneocyte cells surrounded by multilayers of lipids. The lipid layer is responsible for the barrier properties of the skin. We calculate the excess chemical potential and diffusivity of water as a function of depth in lipid bilayers with compositions representative of the stratum corneum using atomistic molecular dynamics simulations. The maximum in the excess free energy of water inside the lipid bilayers is found to be twice that of water in phospholipid bilayers at the same temperature. Permeability, which decreases exponentially with the free energy barrier, is reduced by several orders of magnitude as compared to with phospholipid bilayers. The average time it takes for a water molecule to cross the bilayer is calculated by solving the Smoluchowski equation in presence of the free energy barrier. For a bilayer composed of a 2:2:1 molar ratio of ceramide NS 24:0, cholesterol and free fatty acid 24:0 at 300K, we estimate the permeability P=3.7e-9 cm/s and the average crossing time τ_{av}=0.69 ms. The permeability is about 30 times smaller than existing experimental results on mammalian skin sections.

cond-mat.soft↗

arXiv2009

Simulation studies of stratum corneum lipid mixtures

We present atomistic molecular dynamics results for fully hydrated bilayers composed of ceramide NS-24:0, free fatty acid 24:0 and cholesterol, to address the effect of the different components in the stratum corneum (the outermost layer of skin) lipid matrix on its structural properties. Bilayers containing ceramide molecules show higher in-plane density and hence lower rate of passive transport compared to phospholipid bilayers. At physiological temperatures, for all composition ratios explored, the lipids are in a gel phase with ordered lipid tails. However, the large asymmetry in the lengths of the two tails of the ceramide molecule leads to a fluid like environment at the bilayer mid-plane. The lateral pressure profiles show large local variations across the bilayer for pure ceramide or any of the two component mixtures. Close to the skin composition ratio, the lateral pressure fluctuations are greatly suppressed, the ceramide tails from the two leaflets interdigitate significantly, the depression in local density at the inter-leaflet region is lowered, and the bilayer have lowered elastic moduli. This indicates that the observed composition ratio in the stratum corneum lipid layer is responsible for both the good barrier properties and the stability of the lipid structure against mechanical stresses.

cond-mat.soft↗

arXiv2006

Internal protein dynamics shifts the distance to the mechanical transition state

Mechanical unfolding of polyproteins by force spectroscopy provides valuable insight into their free energy landscapes. Most phenomenological models of the unfolding process are two-state and/or one dimensional, with the details of the protein and its dynamics often subsumed into a zero-force unfolding rate and a single distance $x_u^{1\textrm{D}}$ to the transition state. We consider the entire phase space of a model protein under a constant force, and show that the distance $x_u^{1\textrm{D}}$ contains a sizeable contribution from exploring the full multidimensional energy landscape. Proteins with more degrees of freedom are expected to have larger values for $x_u^{1\textrm{D}}$. We show that externally attached flexible linkers also contribute to the measured unfolding characteristics.

physics.bio-ph↗

arXiv2006

Vorticity Banding During the Lamellar-to-Onion Transition in a Lyotropic Surfactant Solution in Shear Flow

We report on the rheology of a lamellar lyotropic surfactant solution (SDS/dodecane/pentanol/water), and identify a discontinuous transition between two shear thinning regimes which correspond to the low stress lamellar phase and the more viscous shear induced multi-lamellar vesicle, or ``onion'' phase. We study in detail the flow curve, stress as a function of shear rate, during the transition region, and present evidence that the region consists of a shear banded phase where the material has macroscopically separated into bands of lamellae and onions stacked in the vorticity direction. We infer very slow and irregular transformations from lamellae to onions as the stress is increased through the two phase region, and identify distinct events consistent with the nucleation of small fractions of onions that coexist with sheared lamellae.

cond-mat.soft↗