Direct observation of crystallization and aggregation in a phase-separating colloid-polymer suspension

Hoog, E.H.A. de; Kegel, Willem K.; Blaaderen, Alfons van; Lekkerkerker, H. N. W.

doi:10.1103/physreve.64.021407

Cited by 125 publications

(136 citation statements)

References 32 publications

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“…Figure 3a is in reasonable agreement with both theory 19 and experiments 23 for similar q, for crystal-vapour and liquid-vapour phase boundaries in the plane of polymer and colloid volume fractions, f c À f p . A caveat is that these results are subject to the relatively slow nucleation kinetics near the binodal 28 . The resulting configurations were analysed using SteinhardtNelson bond order parameters to characterize crystal grains based on their local orientational symmetry (cf.…”

Section: Resultsmentioning

confidence: 84%

“…First, we consider mixtures of hard-sphere colloids and linear polymers. In the absence of polymer, or for very short chains, hard spheres are experimentally observed to always crystallize into the random hexagonal close-packed (rHCP) structure 4,28 , though computer simulations suggest they may anneal to the more stable FCC polymorph over much longer timescales 7 . In contrast, for polymers long enough to span two (or more) neighbouring crystal voids our simulations show that the HCP crystal is strongly stabilized relative to the FCC, and becomes overwhelmingly the most thermodynamically (and kinetically) favoured state.…”

Section: Resultsmentioning

confidence: 99%

“…External factors such as difficulty in simultaneous density matching of both colloids and depletants 28 and thermal interactions between monomer (Kuhn) segments and the colloids are likely to play an important role. The former is known to affect the kinetics of crystallization in terrestrial environments 8,28 , while the latter can disrupt this effect if the cross-interactions are significant enough; ARTICLE however, the present work illustrates that entropy alone is sufficient to provide a mechanism that can discriminate between polymorphs.…”

Section: Discussionmentioning

confidence: 99%

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Stabilizing colloidal crystals by leveraging void distributions

et al. 2014

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Colloids often crystallize into polymorphic structures, which are only separated by marginal differences in free energy. Due to this fact, the face-centred cubic and hexagonal close-packed hard-sphere morphologies, for example, usually crystallize simultaneously from a supersaturated solution. The resulting lack of long-range order in these polymorphic structures has been a significant barrier to the widespread application of these crystals in, for instance, photonic bandgap materials. Here, we report a simple method to stabilize one out of two competing polymorphs by exploiting the fact that they have significantly different spatial distributions of voids. We use a variety of polymeric additives whose geometries can be tuned such that their entropy loss, which is related to crystal void symmetries, is different in the two competing polymorphs. This, in turn, controls which polymorph is most thermodynamically stable, providing a generalizable means to stabilize a selected crystal polymorph from a suite of competing structures.

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Section: Resultsmentioning

confidence: 84%

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Stabilizing colloidal crystals by leveraging void distributions

et al. 2014

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“…We used polystyrene with M w = 3.0 × 10 7 , which implies a polymer size of 164 nm, and a polymer-colloid size ratio of around 0.25 for a = 650 nm and 0.7 for a = 230 nm in a theta solvent 23 . The larger colloids were dispersed in an almost density-matching solvent of 1:1.015:2.113 tetralin/cis-decalin/carbon tetrachloride by mass 24 . Comparison with free-volume theory 25 suggests a strong degree of polymer swelling, perhaps even to 40%, which, along with the polymer polydispersity (M w /M n = 1.3), should further stabilize the liquid-gas demixing 26 .…”

Section: Methods Experimentalmentioning

confidence: 99%

Bridging length scales in colloidal liquids and interfaces from near-critical divergence to single particles

2007

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Boiling and condensation are among the best recognized phase transitions of condensed matter. Approaching the critical point, a liquid becomes indistinguishable from its vapour, the interfacial thickness diverges and the system is dominated by long-wavelength density fluctuations 1 . Long wavelength usually means hundreds of particle diameters, but here we consider the limits of this assumption, using a mesoscopic analogue of simple liquids, a colloid-polymer mixture 2,3 . We simultaneously visualize both the colloidal particles and near-critical density fluctuations, and reveal particle-level images of the critical clusters and liquid-gas interface. Surprisingly, we find that critical scaling does not break down until the correlation length approaches the size of the constituent particles, where there is a smooth transition to non-critical classical behaviour. Our results could provide a framework for unifying the disparate particle and correlation length scales, and bring new insight into the nature of the liquid-gas interface and the limit of the critical regime.Among the simplest pictures of two different phases is that of a magnet, whose sites can only point up or down, and do so depending solely on their neighbours. This Ising model in fact describes a general class of phase transitions 1 , from the original ferromagnetic transition in magnets to phase separation in alloys, binary liquids, emulsions and polymers, and the liquidgas transition in simple liquids and colloid-polymer mixtures. Although we shall consider the latter two cases, this work has implications for many systems that belong to the same universality class. Close to the critical point, many physical properties such as the bulk correlation length, ξ B , are described by simple scaling, such as ξ B = ξ 0 B ε −ν , where ξ 0 B is the bare correlation length (or critical amplitude) and ε = (T − T C )/T C is a reduced temperature where T is the temperature and T C is the critical temperature. The critical exponent ν = 0.5 according to mean-field theory, and very close to the critical point there is a crossover to ν = 0.63, 'three-dimensional (3D) Ising universality' 4 . We shall refer to the entire scaling regime, both mean field and 3D Ising as the critical region. Although criticality is well understood, the point at which it breaks down has so far received relatively little attention. Conventional theory of critical phenomena assumes a large separation in length scales, that is, that the correlation length is decoupled from other length scales. This length-scale separation lies at the heart of critical universality, implying that the microscopic details are irrelevant. Whereas length-scale coupling has been considered in complex fluids with much longer length scales, such as polymer 5 and ionic 6 solutions, we consider here a more general limit: the case that the correlation length approaches that of the constituent particles. Although it is difficult to approach this limit experimentally in molecular systems, a colloidpolymer mixture provides...

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“…This fraction was set at 0.66 [10]. Confocal scanning laser microscopy was used to image the particles present in the first layer, parallel to the flat glass bottom of the container [11]. No signs of attraction between the particles and the glass wall were found.…”

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confidence: 99%

Reentrant Surface Melting of Colloidal Hard Spheres

Dullens

Kegel

2004

Phys. Rev. Lett.

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Concentrated suspensions of model colloidal hard spheres at a wall were studied in real space by means of time-resolved fluorescence confocal scanning microscopy. Both structure and dynamics of these systems differ dramatically from their bulk analogs (i.e., far away from a wall). In particular, systems that are a glass in the bulk show significant hexagonal order at a wall. Upon increasing the volume fraction of the colloids, a reentrant melting transition involving a hexatic structure is observed. The last observation points to two-dimensional behavior of matter at walls. DOI: 10.1103/PhysRevLett.92.195702 PACS numbers: 64.60.Cn, 64.70.-p, 82.70.Dd Hard spheres under thermal excitation may be considered as the ''fruit flies'' of statistical mechanics, where the hard sphere potential is extensively used as a reference potential [1]. Therefore, rigorous insight into the behavior of hard spheres is imperative for understanding the wide class of systems where excluded volume interactions are important. In nature, all systems are bounded, either by walls or surfaces. These will tend to dominate the behavior as systems become smaller [2 -4]. Motivated by the increasing interest in nanometer-sized systems in science and technology, we address the question as to what surfaces do to the behavior of hard spheres. We expect our results to be relevant for the behavior of many other systems at walls or boundaries in general.Although the role of polydispersity is still being debated, most of the bulk behavior of hard spheres has been well established [5]. In contrast, much less is known about the influence of a single wall on the local structure and dynamics of hard spheres. In this Letter, we use colloidal model particles to address this issue. Colloidal suspensions are extensively used as experimental model hard spheres [5][6][7]. The colloidal model particles used here consist of a core of silica, 450 nm in diameter, labeled with the fluorescent dye fluorescein isothiocyanate [8]. The cores are covered with a thick shell of nonfluorescent polymethylmethacrylate (PMMA). The system is sterically stabilized by a layer of 12-poly-hydroxystearic acid that is covalently linked to the PMMA [9]. The spheres were dispersed in a mixture of tetralin, cis-decalin, and carbon tetrachloride, which (nearly) matches the refractive index and the mass density of the colloids. In this mixture the particles behave as hard spheres [6]. The particle diameter d is 1:4 m and the size polydispersity is 6%. Crystallization was absent in bulk [6].The volume fraction of the samples was defined relative to the volume fraction at random close packing of 6% polydisperse hard spheres. This fraction was set at 0.66 [10]. Confocal scanning laser microscopy was used to image the particles present in the first layer, parallel to the flat glass bottom of the container [11]. No signs of attraction between the particles and the glass wall were found. As the colloidal particles have a core-shell character, time-dependent particle coordinates could be obtai...

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Direct observation of crystallization and aggregation in a phase-separating colloid-polymer suspension

Cited by 125 publications

References 32 publications

Stabilizing colloidal crystals by leveraging void distributions

Stabilizing colloidal crystals by leveraging void distributions

Bridging length scales in colloidal liquids and interfaces from near-critical divergence to single particles

Reentrant Surface Melting of Colloidal Hard Spheres

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