26 research outputs found
Counterion-controlled phase equilibria in a charge-regulated polymer solution
We study phase equilibria in a minimal model of charge-regulated polymer solutions. Our model consists of a single polymer species whose charge state arises from protonation-deprotonation processes in the presence of a dissolved acid, whose anions serve as screening counterions. We explicitly account for variability in the polymers’ charge states. Homogeneous equilibria in this model system are characterised by the total concentration of polymers, the concentration of counter-ions and the charge distributions of polymers which can be computed with the help of analytical approximations. We use these analytical results to characterise how parameter values and solution acidity influence equilibrium charge distributions and identify for which regimes uni-modal and multi-modal charge distributions arise. We then study the interplay between charge regulation, solution acidity and phase separation. We find that charge regulation has a significant impact on polymer solubility and allows for non-linear responses to the solution acidity: Re-entrant phase behaviour is possible in response to increasing solution acidity. Moreover, we show that phase separation can yield to the coexistence of local environments characterised by different charge distributions
Counterion-controlled phase equilibria in a charge-regulated polymer solution
We study phase equilibria in a minimal model of charge-regulated polymer
solutions. Our model consists of a single polymer species whose charge state
arises from protonation-deprotonation processes in the presence of a dissolved
acid, whose anions serve as screening counterions. We explicitly account for
variability in the polymers' charge states. Homogeneous equilibria in this
model system are characterised by the total concentration of polymers, the
concentration of counter-ions and the charge distributions of polymers which
can be computed with the help of analytical approximations. We use these
analytical results to characterise how parameter values and solution acidity
influence equilibrium charge distributions and identify for which regimes
uni-modal and multi-modal charge distributions arise. We then study the
interplay between charge regulation, solution acidity and phase separation. We
find that charge regulation has a significant impact on polymer solubility and
allows for non-linear responses to the solution acidity: re-entrant phase
behaviour is possible in response to increasing solution acidity. Moreover, we
show that phase separation can yield to the coexistence of local environments
characterised by different charge distributions and mixture compositions
Local magneto-shear instability in Newtonian gravity
The magneto-rotational instability (MRI) - which is due to an interplay
between a sheared background and the magnetic field - is commonly considered a
key ingredient for developing and sustaining turbulence in the outer envelope
of binary neutron star merger remnants. To assess whether (or not) the
instability is active and resolved, criteria originally derived in the
accretion disk literature - thus exploiting the symmetries of such systems -
are often used. In this paper we discuss the magneto-shear instability as a
truly local phenomenon, relaxing common symmetry assumptions on the background
on top of which the instability grows. This makes the discussion well-suited
for highly dynamical environments such as binary mergers. We find that -
although this is somewhat hidden in the usual derivation of the MRI dispersion
relation - the instability crucially depends on the assumed symmetries.
Relaxing the symmetry assumptions on the background we find that the role of
the magnetic field is significantly diminished, as it affects the modes' growth
but does not drive it. This suggests that we should not expect the standard
instability criteria to provide a faithful indication/diagnostic of what "is
actually going on" in mergers. We conclude by making contact with a suitable
filtering operation, as this is key to separating background and fluctuations
in highly dynamical systems.Comment: 15 pages, 1 figur
Breakdown of electroneutrality in polyelectrolyte gels
Mathematical models of polyelectrolyte gels are often simplified by assuming the gel is electrically neutral. The rationale behind this assumption is that the thickness of the electric double layer (EDL) at the free surface of the gel is small compared to the size of the gel. Hence, the thin-EDL limit is taken, in which the thickness of the EDL is set to zero. Despite the widespread use of the thin-EDL limit, the solutions in the EDL are rarely computed and shown to match to the solutions for the electrically neutral bulk. The aims of this paper are to study the structure of the EDL and establish the validity of the thin-EDL limit. The model for the gel accounts for phase separation, which gives rise to diffuse interfaces with a thickness described by the Kuhn length. We show that the solutions in the EDL can only be asymptotically matched to the solutions for an electrically neutral bulk, in general, when the Debye length is much smaller than the Kuhn length. If the Debye length is similar to or larger than the Kuhn length, then phase separation can be initiated in the EDL. This phase separation spreads into the bulk of the gel and gives rise to electrically charged layers with different degrees of swelling. Thus, the thin-EDL limit and the assumption of electroneutrality only generally apply when the Debye length is much smaller than the Kuhn length
Asymptotic study of the electric double layer at the interface of a polyelectrolyte gel and solvent bath
An asymptotic framework is developed to study electric double layers that form at the inter-face between a solvent bath and a polyelectrolyte gel that can undergo phase separation. The kinetic model for the gel accounts for the finite strain of polyelectrolyte chains, free energy ofinternal interfaces, and Stefan?Maxwell diffusion. By assuming that the thickness of the doublelayer is small compared to the typical size of the gel, matched asymptotic expansions are used toderive electroneutral models with consistent jump conditions across the gel-bath interface in two-dimensional plane-strain as well as fully three-dimensional settings. The asymptotic frameworkis then applied to cylindrical gels that undergo volume phase transitions. The analysis indicatesthat Maxwell stresses are responsible for generating large compressive hoop stresses in the double layer of the gel when it is in the collapsed state, potentially leading to localised mechanicalinstabilities that cannot occur when the gel is in the swollen state. When the energy cost of in-ternal interfaces is sufficiently weak, a sharp transition between electrically neutral and chargedregions of the gel can occur. This transition truncates the double layer and causes it to have finitethickness. Moreover, phase separation within the double layer can occur. Both of these featuresare suppressed if the energy cost of internal interfaces is sufficiently high. Thus, interfacial freeenergy plays a critical role in controlling the structure of the double layer in the gel
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Spinodal decomposition and collapse of a polyelectrolyte gel
The collapse of a polyelectrolyte gel in a (monovalent) salt solution is analysed using a new model that includes interfacial gradient energy to account for phase separation in the gel, finite elasticity and multicomponent transport. We carry out a linear stability analysis to determine the stable and unstable spatially homogeneous equilibrium states and how they phase separate into localized regions that eventually coarsen to a new stable state. We then investigate the problem of a collapsing gel as a response to increasing the salt concentration in the bath. A phase space analysis reveals that the collapse is obtained by a front moving through the gel that eventually ends in a new stable equilibrium. For some parameter ranges, these two routes to gel shrinking occur together
Spinodal decomposition and collapse of a polyelectrolyte gel
The collapse of a polyelectrolyte gel in a (monovalent) salt solution is analysed using a new model that includes interfacial gradient energy to account for phase separation in the gel, finite elasticity and multicomponent transport. We carry out a linear stability analysis to determine the stable and unstable spatially homogeneous equilibrium states and how they phase separate into localized regions that eventually coarsen to a new stable state. We then investigate the problem of a collapsing gel as a response to increasing the salt concentration in the bath. A phase space analysis reveals that the collapse is obtained by a front moving through the gel that eventually ends in a new stable equilibrium. For some parameter ranges, these two routes to gel shrinking occur together
The Dynamics of a Collapsing Polyelectrolyte Gel
We analyse the dynamics of different routes to collapse of a constrained polyelectrolyte gel in contact with an ionic bath. The evolution of the gel is described by a model that incorporates non-linear elasticity, Stefan-Maxwell diffusion and interfacial gradient free energy to account for phase separation of the gel. A bifurcation analysis of the homogeneous equilibrium states reveals three solution branches at low ion concentrations in the bath, giving way to only one above a critical ion concentration. We present numerical solutions that capture both the spatial heterogeneity and the multiple timescales involved in the process of collapse. These solutions are complemented by two analytical studies. Firstly, a phase-plane analysis that reveals the existence of a depletion front for the transition from the highly swollen to the new collapsed equilibrium state. This depletion front is initiated after the fast ionic diffusion has set the initial condition for this time regime. Secondly, we perform a linear stability analysis about the homogeneous states that show that for a range of ion concentrations in the bath, spinodal decomposition of the swollen state gives rise to localized solvent-rich(poor) and, due to the electroneutrality condition, ion-poor(rich) phases that coarsen on the route to collapse. This dynamics of a collapsing polyelectrolyte gel has not been described before