13 research outputs found
Phase separation dynamics in colloid-polymer mixtures: the effect of interaction range
Colloid-polymer mixtures may undergo either fluid-fluid phase separation or
gelation. This depends on the depth of the quench (polymer concentration) and
polymer-colloid size ratio. We present a real-space study of dynamics in phase
separating colloid-polymer mixtures with medium- to long-range attractions
(polymer-colloid size ratio q_R=0.45-0.89, with the aim of understanding the
mechanism of gelation as the range of the attraction is changed. In contrast to
previous studies of short-range attractive systems, where gelation occurs
shortly after crossing the equilibrium phase boundary, we find a substantial
region of fluid-fluid phase separation. On deeper quenches the system undergoes
a continuous crossover to gel formation. We identify two regimes, `classical'
phase separation, where single particle relaxation is faster than the dynamics
of phase separation, and `viscoelastic' phase separation, where demixing is
slowed down appreciably due to slow dynamics in the colloid-rich phase.
Particles at the surface of the strands of the network exhibit significantly
greater mobility than those buried inside the gel strand which presents a
method for coarsening.Comment: 8 page
Sudden collapse of a colloidal gel
Metastable gels formed by weakly attractive colloidal particles display a
distinctive two-stage time-dependent settling behavior under their own weight.
Initially a space-spanning network is formed that for a characteristic time,
which we define as the lag time \taud, resists compaction. This solid-like
behavior persists only for a limited time. Gels whose age \tw is greater than
\taud yield and suddenly collapse. We use a combination of confocal
microscopy, rheology and time-lapse video imaging to investigate both the
process of sudden collapse and its microscopic origin in an refractive-index
matched emulsion-polymer system. We show that the height of the gel in the
early stages of collapse is well described by the surprisingly simple
expression, h(\ts) = \h0 - A \ts^{3/2}, with \h0 the initial height and
\ts = \tw-\taud the time counted from the instant where the gel first yields.
We propose that this unexpected result arises because the colloidal network
progressively builds up internal stress as a consequence of localized
rearrangement events which leads ultimately to collapse as thermal equilibrium
is re-established.Comment: 14 pages, 11 figures, final versio
Gels under stress: the origins of delayed collapse
Attractive colloidal particles can form a disordered elastic solid or gel
when quenched into a two-phase region, if the volume fraction is sufficiently
large. When the interactions are comparable to thermal energies the
stress-bearing network within the gel restructures over time as individual
particle bonds break and reform. Typically, under gravity such weak gels show a
prolonged period of either no or very slow settling, followed by a sudden and
rapid collapse - a phenomenon known as delayed collapse. The link between local
bond breaking events and the macroscopic process of delayed collapse is not
well understood. Here we summarize the main features of delayed collapse and
discuss the microscopic processes which cause it. We present a plausible model
which connects the kinetics of bond breaking to gel collapse and test the model
by exploring the effect of an applied external force on the stability of a gel.Comment: Accepted version: 10 pages, 7 figure
The role of initiator on the dispersibility of poly(styrene) microgels in non-aqueous solvents
Non-aqueous microgel particles are commonly synthesised in water, dried, and then redispersed in non-aqueous solvents. An important factor to consider when synthesising such particles is the initiator, which can determine how well the particles disperse in solvents. Polystyrene microgel particles were made with three different initiators. When a neutral, oil soluble initiator (azobisisobutyronitrile) was used the particles dispersed in toluene as well as cyclohexane and decalin. In contrast, anionic, water-soluble initiators (potassium persulfate or azobis(4-cyanovaleric acid)) created particles that only redispersed in toluene and not the other two solvents. Of the three considered, toluene is the best solvent for polystyrene and also has the highest polarizability, making it most effective at redispersing particles with polar or ionisable functional groups. Zeta potential and conductivity measurements, however, did not detect a direct relationship between particle charging and redispersibility. Oil soluble initiators result in “inside out” polymerisation where the initiator groups are buried inside the growing particle, whereas water-soluble initiators result in “outside in” polymerisation, with the polar initiator groups residing on the particle surface. By tailoring the ratio between water and oil soluble initiators, it may be possible to synthesise microgel particles with uniform or designed charge profiles from the core to the surface. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00396-017-4023-y) contains supplementary material, which is available to authorized users
Real space analysis of colloidal gels: triumphs, challenges and future directions
Colloidal gels constitute an important class of materials found in many contexts and with a wide range of applications. Yet as matter far from equilibrium, gels exhibit a variety of time-dependent behaviours, which can be perplexing, such as an increase in strength prior to catastrophic failure. Remarkably, such complex phenomena are faithfully captured by an extremely simple model—'sticky spheres'. Here we review progress in our understanding of colloidal gels made through the use of real space analysis and particle resolved studies. We consider the challenges of obtaining a suitable experimental system where the refractive index and density of the colloidal particles is matched to that of the solvent. We review work to obtain a particle-level mechanism for rigidity in gels and the evolution of our understanding of time-dependent behaviour, from early-time aggregation to ageing, before considering the response of colloidal gels to deformation and then move on to more complex systems of anisotropic particles and mixtures. Finally we note some more exotic materials with similar properties
Composition inversion in mixtures of binary colloids and polymer
Understanding the phase behaviour of mixtures continues to pose challenges,
even for systems that might be considered "simple". Here we consider a very
simple mixture of two colloidal and one non-adsorbing polymer species which can
be simplified even further to a size-asymmetrical binary mixture, in which the
effective colloid-colloid interactions depend on the polymer concentration. We
show that this basic system exhibits surprisingly rich phase behaviour. In
particular, we enquire whether such a system features only a liquid-vapor phase
separation (as in one-component colloid-polymer mixtures) or whether,
additionally, liquid-liquid demixing of two colloidal phases can occur.
Particle-resolved experiments show demixing-like behaviour, but when combined
with bespoke Monte Carlo simulations, this proves illusory, and we reveal that
only a single liquid-vapor transition occurs. Progressive migration of the
small particles to the liquid phase as the polymer concentration increases
gives rise to composition inversion - a maximum in the large particle
concentration in the liquid phase. Near criticality the density fluctuations
are found to be dominated by the larger colloids.Comment: submitted to J. Chem. Phy
Composition inversion in mixtures of binary colloids and polymer
Understanding the phase behaviour of mixtures continues to pose challenges, even for systems that might be considered "simple." Here, we consider a very simple mixture of two colloidal and one non-adsorbing polymer species, which can be simplified even further to a size-asymmetrical binary mixture, in which the effective colloid-colloid interactions depend on the polymer concentration. We show that this basic system exhibits surprisingly rich phase behaviour. In particular, we enquire whether such a system features only a liquid-vapor phase separation (as in one-component colloid-polymer mixtures) or whether, additionally, liquid-liquid demixing of two colloidal phases can occur. Particle-resolved experiments show demixing-like behaviour, but when combined with bespoke Monte Carlo simulations, this proves illusory, and we reveal that only a single liquid-vapor transition occurs. Progressive migration of the small particles to the liquid phase as the polymer concentration increases gives rise to composition inversion - a maximum in the large particle concentration in the liquid phase. Close to criticality, the density fluctuations are found to be dominated by the larger colloids.</p