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

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

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 $h$ 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

Opposed flow focusing: evidence of a second order jetting transition

We propose a novel microfluidic "opposed-flow" geometry in which the continuous fluid phase is fed into a junction in a direction opposite the dispersed phase. This pulls out the dispersed phase into a micron-sized jet, which decays into micron-sized droplets. As the driving pressure is tuned to a critical value, the jet radius vanishes as a power law down to sizes below 1 $\mu$m. By contrast, the conventional "coflowing" junction leads to a first order jetting transition, in which the jet disappears at a finite radius of several $\mu$m, to give way to a "dripping" state, resulting in much larger droplets. We demonstrate the effectiveness of our method by producing the first microfluidic silicone oil emulsions with a sub micron particle radius, and utilize these droplets to produce colloidal clusters

Interaction forces between particles stabilized by a hydrophobically modified inulin surfactant.

Submitted versio

Binary and ternary mixtures of microgel particles, hard spheres and non-adsorbing polymer in non-aqueous solvents

Crosslinked polystyrene (PS) particles were dispersed in diisopropyl adipate, a non-volatile, good solvent for PS. Depletion attractions between particles were induced by adding linear PS, with a polymer/particle size ratio of 0.3. For hard particles, with a high crosslink density, the colloid-polymer mixtures displayed phase separation in agreement with predictions for hard sphere - polymer mixtures. For similar sized but weakly crosslinked, soft microgel particles however, a significantly higher concentration of linear PS needed to be added to observe phase separation. This is because the non-adsorbing polymer can penetrate the soft microgels which weakens the depletion interaction.In ternary mixtures of the hard and soft particles and linear PS, confocal microscopy reveals that mixed particle networks are formed. Upon adding soft particles to a hard particle gel network, there is only modest variation in the flow properties, with the moduli decreasing somewhat, yet viscosity increasing. It is argued that the effect of an increase of volume fraction is offset by a reduction in depletion interaction strength. This demonstrates that, in these ternary mixtures, microgels act like particles rather than polymer depletants; yet microgel addition allows high volume fraction polymer - colloid mixtures to be made whilst avoiding the viscosity increase that would normally result

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