arXiv subjects

Eugene M. Terentjev

Publications and source records attributed to Eugene M. Terentjev.

At least 19 recordsLinked to original sources

arXiv2026

Properties of Liquid Crystalline Elastomer Foams

We investigate how controlled foaming alters the mechanical dissipation of liquid crystalline elastomers (LCEs). Using thermal expandable microspheres, we generate homogeneous foams with precisely tuned bubble volume fractions up to 13% and compare their behaviour with non-mesogenic silicon analogues. We show that microsphere expansion induces a particle-centred mesogenic interphase arising from local elastic distortion and preferential alignment of mesogenic units at the inclusion surface. At low bubble volume fraction (0.5 to 5%), these interfaces remain spatially isolated and produce a pronounced no-monotonic enhancement of damping, with the loss factor reaching tan-delta=0.2 even in the isotropic regime. At higher loading, interphase overlaps and mechanical constraints suppress this effect, and the dissipation returns towards baseline elastomeric values. Large-strain tensile tests and impact experiments exhibit the same non-monotonic trend, demonstrating that low density LCE forms achieve the highest mechanical energy absorption per unit mass. Compared with conventional high porosity polymer foams used for acoustic damping, these materials retain sufficient mechanical integrity to sustain impact loads, establishing a microstructural route to engineer high-performance damping in soft solids.

cond-mat.soft↗

arXiv2025

Free energy of self-avoiding polymer chain confined between parallel walls

Understanding and computing the entropic forces exerted by polymer chains under confinement is important for many reasons, from research to applications. However, extracting properties related to the free energy, such as the force (or pressure) on confining walls, does not readily emerge from conventional polymer dynamics simulations due to the entropic contributions inherent in these free energies. Here we propose an alternative method to compute such forces, and the associated free energies, based on empirically measuring the average force required to confine a polymer chain between parallel walls connected by an artificial elastic spring. This measurement enables us to interpolate the expression for the free energy of a confined self-avoiding chain and offer an analytical expression to complement the classical theory of ideal chains in confined spaces. Therefore, the significance of our method extends beyond the findings of this paper: it can be effectively employed to investigate the confinement free energy across diverse scenarios where all kinds of polymer chains are confined in a gap between parallel walls.

cond-mat.soft↗

arXiv2023

Mechanism of pressure sensitive adhesion in nematic elastomers

Nematic liquid crystal elastomers (LCEs) have anomalously high vibration damping, and it has been assumed this is the cause of their anomalously high pressure-sensitive adhesion (PSA). Here we investigate the mechanism behind this enhanced PSA by first preparing thin adhesive tapes with LCE of varying crosslinking density, characterizing their material and surface properties, and then studying the adhesion characteristics with a standard set of 90-deg peel, lap shear, and probe tack tests. The study confirms that the enhanced PSA is only present in (and due to) the nematic phase of the elastomer, and the strength of bonding takes over 24 hours to fully reach its maximum value. Such a long saturation time is caused by the slow relaxation of local stress and director orientation in nematic domains after pressing against the surface. We confirm this mechanism by showing that a freshly pressed and annealed tape reaches the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film.

cond-mat.soft↗

arXiv2023

Softening of the Hertz indentation contact in nematic elastomers

Polydomain liquid crystalline (nematic) elastomers have highly unusual mechanical properties, dominated by the dramatically non-linear stress-strain response that reflects stress-induced evolution of domain patterns. Here, we study the classical Hertz indentation problem in such a material. Experimentally, we find that polydomain nematic elastomers display a smaller exponent than the classical 3/2 in the load vs. indentation depth response. This is puzzling: asymptotically a softer stress-strain response requires a larger exponent at small loads. We resolve this by theory where three regimes are identified -- an initial elastic regime for shallow indentation that is obscured in experiment, an intermediate regime where local domain pattern evolution leads to a smaller scaling in agreement with experiments, and a final stiffening regime where the completion of local domain evolution returns the response to elastic. This three-regime structure is universal, but the intermediate exponent is not. We discuss how our work reveals a new mechanism of enhanced adhesion for pressure-sensitive adhesion of nematic elastomers.

cond-mat.soft↗

arXiv2022

Nucleation of cadherin clusters on cell-cell interfaces

Cadherins mediate cell-cell adhesion and help the cell determine its shape and function. Here we study collective cadherin organization and interactions within cell-cell contact areas, and find the cadherin density at which a gas-liquid phase transition occurs, when cadherin monomers begin to aggregate into dense clusters. We use a 2D lattice model of a cell-cell contact area, and coarse-grain to the continuous number density of cadherin to map the model onto the Cahn-Hilliard coarsening theory. This predicts the density required for nucleation, the characteristic length scale of the process, and the number density of clusters. The analytical predictions of the model are in good agreement with experimental observations of cadherin clustering in epithelial tissues.

cond-mat.soft↗

arXiv2021

Why exercise builds muscles: Titin mechanosensing controls skeletal muscle growth under load

Muscles sense internally generated and externally applied forces, responding to these in a coordinated hierarchical manner at different time scales. The center of the basic unit of the muscle, the sarcomeric M-band, is perfectly placed to sense the different types of load to which the muscle is subjected. In particular, the kinase domain (TK) of titin located at the M-band is a known candidate for mechanical signaling. Here, we develop the quantitative mathematical model that describes the kinetics of TK-based mechanosensitive signaling, and predicts trophic changes in response to exercise and rehabilitation regimes. First, we build the kinetic model for TK conformational changes under force: opening, phosphorylation, signaling and autoinhibition. We find that TK opens as a metastable mechanosensitive switch, which naturally produces a much greater signal after high-load resistance exercise than an equally energetically costly endurance effort. Next, in order for the model to be stable, give coherent predictions, in particular the lag following the onset of an exercise regime, we have to account for the associated kinetics of phosphate (carried by ATP), and for the non-linear dependence of protein synthesis rates on muscle fibre size. We suggest that the latter effect may occur via the steric inhibition of ribosome diffusion through the sieve-like myofilament lattice. The full model yields a steady-state solution (homeostasis) for muscle cross-sectional area and tension, and a quantitatively plausible hypertrophic response to training as well as atrophy following an extended reduction in tension.

q-bio.TO↗

arXiv2020

Rheology of Hard Glassy Materials

Glassy solids may undergo a fluidization (yielding) transition upon deformation whereby the material starts to flow plastically. It has been a matter of debate whether this process is controlled by a specific time scale, from among different competing relaxation/kinetic processes. Here, two constitutive models of cage relaxation are examined within the microscopic model of nonaffine elasto-plasticity. One (widely used) constitutive model implies that the overall relaxation rate is dominated by the fastest between the structural ($α$) relaxation rate and the shear-induced relaxation rate. A different model is formulated here which, instead, assumes that the slowest (global) relaxation process controls the overall relaxation. We show that the first model is not compatible with the existence of finite elastic shear modulus for quasistatic (low-frequency) deformation, while the second model is able to describe all key features of deformation of `hard' glassy solids, including the yielding transition, the nonaffine-to-affine plateau crossover, and the rate-stiffening of the modulus. The proposed framework provides an operational way to distinguish between `soft' glasses and `hard' glasses based on the shear-rate dependence of the structural relaxation time.

cond-mat.soft↗

arXiv2020

Rates of transesterication in epoxy-thiol vitrimers

Vitrimers, an important subset of dynamically crosslinked polymer networks, have many technological applications for their excellent properties, and the ability to be re-processed through plastic flow above the so-called vitrification temperature. We report a simple and efficient method of generating such adaptive crosslinked networks relying on transesterification for their bond exchange by utilising the `click' chemistry of epoxy and thiols, which also has the advantage of a low glass transition temperature. We vary the chemical structure of thiol spacers to probe the effects of concentration and the local environment of ester groups on the macroscopic elastic-plastic transition. The thermal activation energy of transesterification bond exchange is determined for each chemical structure, and for a varying concentration of catalyst, establishing the conditions for the optimal, and for the suppressed bond exchange. However, we also discover that the temperature of elastic-plastic transition is strongly affected by the stiffness (dynamic rubber modulus) of the network, with softer networks having a much lower vitrification temperature even when their bond-exchange activation energy is higher. This combination of chemical and physical control factors should help optimise the processability of vitrimer plastics.

cond-mat.soft↗

arXiv2019

Dynamic manipulation of friction in smart textile composites of liquid-crystal elastomers

Smart surfaces that reversibly change the interfacial friction coefficients in response to external stimuli enable a wide range of applications, such as grips, seals, brake pads, packaging films, and fabrics. Here a new concept of such a smart frictional system is reported: a composite film of a plain-weave polyester textile sheet, and a thermo-responsive nematic liquid crystalline elastomer (LCE). The composite is deployed with retractable micro-undulations of the elastomer inside each weave mesh, enabling dramatic changes of the contact interface with the opposing surface on LCE actuation, which is induced e.g. by a change in temperature (T). At ambient T, the protruding viscoelastic parts of LCE in the nematic phase make contact with the opposing flat surface, resulting in a very high friction. At an elevated T (50C, isotropic phase), the undulations of LCE surface are retracted within the thickness of the textile, and the contacts are limited to small regions around overlapping textile fibers, lowering the friction dramatically. This effect is fully reversible on heating/cooling cycles. The surface undulations are spontaneous, i.e. fabricated without any lithographic or alignment techniques. The present composite opens a new way to practical uses of sheets/films with switchable friction enabled by stimuli-responsive LCEs.

physics.app-ph↗

arXiv2019

FliI6-FliJ molecular motor assists with unfolding in the type III secretion export apparatus

The role of rotational molecular motors of the ATP synthase class is integral to the metabolism of cells. Yet the function of FliI6-FliJ complex - homologous to the F1 ATPase motor - within the flagellar export apparatus remains unclear. We use a simple two-state model adapted from studies of linear molecular motors to identify key features of this motor. The two states are the 'locked' ground state where the FliJ coiled coil filament experiences fluctuations in an asymmetric torsional potential, and a 'free' excited state in which FliJ undergoes rotational diffusion. Michaelis-Menten kinetics was used to treat transitions between these two states, and obtain the average angular velocity of the FliJ filament within the FliI6 stator: ~9.0 rps. The motor was then studied under external counter torque conditions in order to ascertain its maximal power output: Pmax ~42 kBT/s, and the stall torque: ~3 kBT/rad. Two modes of action within the flagellar export apparatus are proposed, in which the motor performs useful work either by continuously 'grinding' through the resistive environment, or by exerting equal and opposite stall force on it. In both cases, the resistance is provided by flagellin subunits entering the flagellar export channel prior to their unfolding. We therefore propose that the function of the FliI6-FliJ complex is to lower the energy barrier and therefore assist in unfolding of the flagellar proteins before feeding them into the transport channel.

q-bio.SC↗

arXiv2019

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Liquid crystalline elastomers (LCE) undergo reversible shape changes in response to stimuli, which may enable a wide range of smart applications, such as soft robot, adhesive systems or implantable medical devices. Here we introduce new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCE to enable processing (director alignment, remolding, and welding). Unlike, the traditional siloxane-based LCE, which are produced by a other reaction schemes with irreversible bonds (e.g. hydrosilylation), here we use a much more robust reaction (thiol-acrylate/thiol-ene 'double-click' chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces LCEs with tunable properties, low glass transition (-30C), controllable nematic to isotropic transition (32 to 75C), and a very high vitrification temperature (250C). Accordingly, this class of dynamically crosslinked LCE shows unprecedented thermal stability within the working temperature range (-50 to 140C), over many thermal actuation cycles without any creep. Finally, multiple materials sharing the same siloxane exchangeable bonds can be welded into single structures to allow for materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.

cond-mat.mtrl-sci↗

arXiv2019

Stimulated binding of polymer chains by narrow tube confinement

In biology, there are several processes in which unfolded protein chains are transported along narrow-tube channels. Normally, without such a severe configurational constraint, unfolded polypeptides would not bind to each other. However, when chain entropy is much reduced in the narrow channel, we find that polypeptide chains have a propensity to bind, even if there is no great potential energy gain in doing so. We find the average length of binding m* (the number of monomers at the chain ends that form bonds) and the critical tube diameter at which such constrained binding occurs. We carry out Brownian dynamics simulations of tightly confined chains, demonstrating their binding over the characteristic length m*, changing in tubes of different diameter.

cond-mat.soft↗

arXiv2018

Universal kinetics for engagement of mechanosensing pathways in cell adhesion

When plated onto substrates, cell morphology and even stem cell differentiation are influenced by the stiffness of their environment. Stiffer substrates give strongly spread (eventually polarized) cells with strong focal adhesions, and stress fibers; very soft substrates give a less developed cytoskeleton, and much lower cell spreading. The kinetics of this process of cell spreading is studied extensively, and important universal relationships are established on how the cell area grows with time. Here we study the population dynamics of spreading cells, investigating the characteristic processes involved in cell response to the substrate. We show that unlike the individual cell morphology, this population dynamics does not depend on the substrate stiffness. Instead, a strong activation temperature dependence is observed. Different cell lines on different substrates all have long-time statistics controlled by the thermal activation over a single energy barrier dG=19 kcal/mol, while the early-time kinetics follows a power law $t^5$. This implies that the rate of spreading depends on an internal process of adhesion-mechanosensing complex assembly and activation: the operational complex must have 5 component proteins, and the last process in the sequence (which we believe is the activation of focal adhesion kinase) is controlled by the binding energy dG.

q-bio.SC↗

arXiv2017

Reaching to a small target: entropic barriers and rates of specific binding of polymer to substrate

This paper considers a broadly biologically relevant question of a chain (such as a protein) binding to a sequence of receptors with matching multiple ligands distributed along the chain. This binding is critical in cell adhesion events, and in protein self-assembly. Using a mean field approximation of polymer dynamics, we first calculate the characteristic binding time for a tethered ligand reaching for a specific binding site on the surface. This time is determined by two separate entropic effects: an entropic barrier for the chain to be stretched sufficiently to reach the distant target, and a restriction on chain conformations near the surface. We then derive the characteristic time for a sequence of single binding events, and find that it is determined by the `zipper effect', optimizing the sequence of single and multiple binding steps.

cond-mat.soft↗

arXiv2017

Mechanisms and Rates of Nucleation of Amyloid Fibrils

The classical nucleation theory finds the rate of nucleation proportional to the monomer concentration raised to the power, which is the `critical nucleaus size', ${n_c}$. The implicit assumption, that amyloids nucleate in the same way, has been recently challenged by an alternative two-step mechanism, when the soluble monomers first form a metastable aggregate (micelle), and then undergo conversion into the conformation rich in $β$-strands that are able to form a stable growing nucleus for the protofilament. Here we put together the elements of extensive knowledge about aggregation and nucleation kinetics, using a specific case of A${β_{1\mathrm{-}42}}$ amyloidogenic peptide for illustration, to find theoretical expressions for the effective rate of amyloid nucleation. We find that at low monomer concentration in solution, and also at low interaction energy between two peptide conformations in the micelle, the nucleation occurs via the classical route. At higher monomer concentration, and a range of other interaction parameters between peptides, the two-step `aggregation-conversion' mechanism of nucleation takes over. In this regime, the effective rate of the process can be interpreted as a power of monomer concentration in a certain range of parameters, however, the exponent is determined by a complicated interplay of interaction parameters and is not related to the minimum size of the growing nucleus (which we find to be ${\sim}$ 7-8 for A${β_{1-42}}$).

cond-mat.soft↗

arXiv2017

Focal adhesion kinase - the reversible molecular mechanosensor

Sensors are the first element of the pathways that control the response of cells to their environment. After chemical, the next most important cue is mechanical, and protein complexes that produce or enable a chemical signal in response to a mechanical stimulus are called mechanosensors. There is a sharp distinction between sensing an external force or pressure/tension applied to the cell, and sensing the mechanical stiffness of the environment. We call the first mechanosensitivity of the 1st kind, and the latter mechanosensitivity of the 2nd kind. There are two variants of protein complexes that act as mechanosensors of the 2nd kind: producing either a one-off or a reversible action. The latent complex of TGF-$β$ is an example of the one-off action: on the release of active TGF-$β$ signal, the complex is discarded and needs to be replaced. In contrast, focal adhesion kinase (FAK) in a complex with integrin is a reversible mechanosensor, which initiates the chemical signal in its active phosphorylated conformation, but can spontaneously return to its closed folded conformation. Here we study the physical mechanism of the reversible mechanosensor of the 2nd kind, using FAK as a practical example. We find how the rates of conformation changes depend on the substrate stiffness and the pulling force applied from the cell cytoskeleton. The results compare well with the phenotype observations of cells on different substrates.

q-bio.SC↗

arXiv2016

Polymer glass transition occurs at the marginal rigidity point with connectivity z*=4

We re-examine the physical origin of the polymer glass transition from the point of view of marginal rigidity, which is achieved above a certain number of intermolecular contacts. In the case of polymer chains in a melt / poor solvent, each monomer has two neighbors bound by covalent bonds and also a number of central-force contacts modelled by the Lennard-Jones (LJ) potential. We find that when the average number of contacts per monomer (covalent and non-covalent) exceeds the critical value z*=4, the system becomes solid and the dynamics arrested - a state that we declare the glass. Coarse-grained Brownian dynamics simulations show that at sufficient strength of LJ attraction (which effectively represents the depth of quenching, or the quality of solvent) the polymer globule indeed crosses the threshold of z*=4, and does become a glass. We verify this by showing the distinction between the `liquid' polymer droplet at z z*, which retains its shape frozen at the moment of z* crossover. These results provide a robust working criterion to tell the liquid apart from the glass for the linear polymers.

cond-mat.soft↗

arXiv2016

Nonlinear elasticity of semiflexible filament networks

We develop a continuum theory for equilibrium elasticity of a network of crosslinked semiflexible filaments, spanning the full range between flexible entropy-driven chains to stiff athermal rods. We choose the 3-chain constitutive model over several plausible candidates, and derive analytical expressions for the elastic energy at arbitrary strain, with the corresponding stress-strain relationship. The theory fits well to a wide range of experimental data on simple shear in different filament networks, quantitatively matching the differential shear modulus variation with stress, with only two adjustable parameters (which represent the filament stiffness and pre-tension in the network, respectively). The general theory accurately describes the crossover between the positive and negative Poynting effect (normal stress on imposed shear) on increasing the stiffness of filaments forming the network.

cond-mat.soft↗