[Show abstract][Hide abstract] ABSTRACT: Upon demixing, an aqueous solution of a polyelectrolyte and an incompatible neutral polymer yields two phases separated by an interface with an ultralow tension. Here, both in theory and experiment, we study this interfacial tension in detail: how it scales with the concentrations of the polymers in the two phases and how it is affected by the interfacial difference in the electrical potential. Experiments are performed on an aqueous model system of uncharged dextran and charged nongelling gelatin. The experimental tension scales to the power ∼3 with the tie-line length in the phase diagram of demixing, in agreement with mean-field theory where space is filled with a binary mixture of polymer blobs. The interfacial electrical potential difference is experimentally found to decrease the interfacial tension in a way that is consistent with Poisson−Boltzmann theory inspired from Frenkel and Verwey−Overbeek.
[Show abstract][Hide abstract] ABSTRACT: Electric charge at the water-water interface of demixed solutions of neutral polymer and polyelectrolyte decreases the already ultralow interfacial tension. This is demonstrated in experiments on aqueous mixtures of dextran (neutral) and nongelling fish gelatin (charged). Upon phase separation, electric charge and a potential difference develop spontaneously at the interface, decreasing the interfacial tension purely electrostatically in a way that can be accounted for quantitatively by Poisson-Boltzmann theory. Interfacial tension is a key property when it comes to manipulating the water-water interface, for instance to create novel water-in-water emulsions.
[Show abstract][Hide abstract] ABSTRACT: The phase behaviour of mixed suspensions of large and small hard platelets is treated by osmotic equilibrium theory. Depending on the size ratios of the small and large platelets, three distinct phase behaviour scenarios appear: isotropic–isotropic–nematic, isotropic–nematic and isotropic–nematic–nematic coexistence regions, in agreement with computer simulations. The experimental multiphase coexistence observed in mixed suspensions of large Titanate platelets and small Laponite platelets can be reproduced semi-quantitatively using osmotic equilibrium theory calculations.
[Show abstract][Hide abstract] ABSTRACT: Smectite clay minerals and their suspensions have long been of both great scientific and applications interest and continue to display a remarkable range of new and interesting behaviour. Recently there has been an increasing interest in the properties of mixed suspensions of such clays with nanoparticles of different size, shape and charge. This review aims to summarize the current status of research in this area focusing on phase behaviour and rheological properties. We will emphasize the rich range of data that has emerged for these systems and the challenges they present for future investigations The review starts with a brief overview of the behaviour and current understanding of pure smectite clays and their suspensions. We then cover the work on smectite clay-inorganic nanoparticle mixed suspensions according to the shape and charge of the nanoparticles – spheres, rods and plates either positively or negatively charged. We conclude with a summary of the overarching trends that emerge from these studies and indicate where gaps in our understanding need further research for better understanding the underlying chemistry and physics.
[Show abstract][Hide abstract] ABSTRACT: In this article we present a study of the structure and rheology of mixed suspensions of Montmorillonite clay platelets and Ludox TMA silica spheres at pH 5, 7 and 9. Using cryogenic transmission electron microscopy (cryo-TEM) we probe the changes in the structure of the montmorillonite suspensions induced by changing the pH and by adding silica particles. Using oscillatory and transient rheological measurements we examine the changes in storage modulus and yield stress of the montmorillonite suspensions upon changing the pH and adding silica particles. Cryo-TEM images reveal that changes in pH have a significant effect on the structure of the suspensions, which can be related to the change in charge of the edges from positive at pH 5 to negative at higher pH. Furthermore, at pH 7, the cryo-TEM images show indications of a micro phase separation between clay and silica particles. The addition of silica leads to lowering of the storage modulus and yield stress, which we connect to the structural changes of the suspension.
The Journal of Physical Chemistry B 09/2014; 118(40). DOI:10.1021/jp504217m · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have studied the effect of additions of both anionic and cationic spherical silica colloids of different sizes on the rheology of dispersions of a well-characterised montmorillonite clay, SWy-2. The systems have been studied above and below the critical hydrodynamic overlap concentration, c*, of the clay. For dispersions at c < c* on replacement of similar to 10 % w/w of the clay content by silica, it was found that whereas a cationic silica additive transformed a liquid-like, non-gelling montmorillonite dispersion into a substantial gel, anionic silica destroyed any nascent structure in the fluids, reducing the effective viscosity and virtually eliminating the rheological hysteresis characteristic of structured fluids. On the other hand, in the regime of c > c*, replacement of similar to 10 % w/w of the clay content by silica leads to enhancements of all the rheological parameters characteristic of a gelling system, for the addition of both anionic and cationic silica. A simple tentative microstructural model for this complex behaviour is presented. This work, alongside our previous studies, confirms significant rheological modification by the addition of small quantities of nanoparticles as a general phenomenon of clay-colloid systems. It further suggests that viscosity enhancement and control of the rates of sol-gel transitions for product applications can be achieved using relatively low-cost, commercially available materials, such as silica nanoparticles and natural clays of different mineralogy.
[Show abstract][Hide abstract] ABSTRACT: In this article we present a study of the liquid crystal phase behaviour of mixed suspensions of the natural smectite clay mineral beidellite and non-adsorbing colloidal silica particles. While virtually all smectite clays dispersed in water form gels at very low concentrations, beidellite displays a first order isotropic-nematic phase transition before gel formation (E. Paineau et al. J. Phys. Chem. B, 2009, 113, 15858-15869). The addition of silica nanospheres shifts the concentrations of the coexisting isotropic and nematic phases to slightly higher values while at the same time markedly accelerating the phase separation process. Furthermore beidellite suspensions at volume fractions above the isotropic-nematic phase separation, trapped in a kinetically arrested gel state, liquefy on the addition of silica nanospheres and proceed to isotropic-nematic phase separation. Using small-angle X-ray scattering (SAXS), we probe the structural changes caused by the addition of the silica nanospheres and we relate the modification of the phase transition kinetics to the change of the rheological properties.
The Journal of Physical Chemistry B 04/2014; 118(18). DOI:10.1021/jp500036v · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Computer simulations, theoretical investigations and experiments carried out over the last 20 years have demonstrated that suspensions of hard plate-like colloids display a rich liquid-crystal phase behaviour including nematic and columnar phases. Recently, it has become clear that charged colloidal platelets display an even richer phase behaviour including smectic A and smectic B phases. Here, we report on the formation of liquid crystals in suspensions of charged silica-coated gibbsite platelets in dimethylformamide in the presence of 1 mM salt and without added salt. A nematic phase is observed in the system with added salt, while in the system without added salt a lamellar (smectic A) liquid-crystalline phase is formed as demonstrated by optical observations and synchrotron small angle X-ray scattering measurements. The observed phases are in contrast with the columnar phase that would be expected if the platelets were interacting through hard-core repulsions. A bifurcation analysis suggests that, on decreasing ionic strength, first the columnar phase is destabilised in favour of nematic order, which in turn is destabilised in favour of lamellar order. Our experimental and theoretical results support the suggestion that the appearance of nematic and lamellar phases is related to the strength and range of the double-layer repulsion.
[Show abstract][Hide abstract] ABSTRACT: This mini-review discusses the influence of external fields on the phase behaviour of lyotropic colloidal liquid crystals. The liquid crystal phases reviewed, formed in suspensions of highly anisotropic particles ranging from rod-to board-to plate-like particles, include ne-matic, smectic and columnar phases. The external fields considered are the earth gravitational field and electric and magnetic fields. For elec-tric and magnetic fields single particle alignment, collective reorienta-tion behaviour of ordered phases and field-induced liquid crystal phase transitions are discussed. Additionally, liquid crystal phase behaviour in various confining geometries, e.g. slit-pore, circular and spherical confinement will be reviewed.
The European Physical Journal Special Topics 11/2013; 222(11):3053-3069. DOI:10.1140/epjst/e2013-02075-x · 1.40 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A review is given of the field of mineral colloidal liquid crystals: liquid crystal phases formed by individual mineral particles within colloidal suspensions. Starting from their discovery in the 1920s, we discuss developments on the levels of both fundamentals and applications. We conclude by highlighting some promising results from recent years, which may point the way towards future developments.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 04/2013; 371(1988):20120263. DOI:10.1098/rsta.2012.0263 · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a study on the macroscopic, microscopic, and rheological behavior of mixtures of natural hectorite clay and different types of anionic Ludox silica spheres. Adding silica spheres to the weak hectorite gels leads the collapse of the suspensions, while the strong gels remain space-filling, though their storage modulus and the yield stress values diminish. We discuss what kind of structural rearrangements are possibly responsible for the macroscopic and rheological changes in the clay/silica mixtures.
The Journal of Physical Chemistry B 07/2012; 116(31):9532-9. DOI:10.1021/jp302472e · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: While the columnar liquid crystalline phase in suspensions of plate-like colloids is by now well-established, little is known about the pathway leading to the formation of this highly ordered, self-assembled structure. Here, we present direct observations of the morphology and structure of micrometer-sized droplets of the columnar phase formed in the nematic phase in suspensions of colloidal gibbsite plates. From polarized light microscopy and optical Bragg reflection measurements we deduce that these droplets consist of stacks of platelets in a hexagonal arrangement, forming a disk-shaped droplet. We discuss the relation of this droplet structure to the nucleation pathway of the columnar phase and to the anisotropic nematic–columnar interfacial tension.
[Show abstract][Hide abstract] ABSTRACT: Here, we present the first observation of a smectic B (Sm(B)) phase in a system of charged colloidal gibbsite platelets suspended in dimethyl sulfoxide (DMSO). The use of DMSO, a polar aprotic solvent, leads to a long range of the electrostatic Coulomb repulsion between platelets. We believe this to be responsible for the formation of the layered liquid crystalline phase consisting of hexagonally ordered particles, that is, the Sm(B) phase. We support our finding by high-resolution X-ray scattering experiments, which additionally indicate a high degree of ordering in the Sm(B) phase.
Journal of the American Chemical Society 03/2012; 134(13):5985-90. DOI:10.1021/ja300527w · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied the snap-off of nematic liquid crystalline droplets originating from the Rayleigh–Taylor instability at the isotropic–nematic interface in suspensions of charged gibbsite in water and sterically stabilized gibbsite in bromotoluene. We found that droplet snap-off strongly depends on the director field structure inside the thinning neck, which is determined by the ratio of the splay elastic constant and the anchoring strength of the nematic phase to the droplet interface relative to the thickness of the thinning neck. If anchoring is weak, which is the case for aqueous gibbsite, this ratio is comparable to the thickness of the breaking thread. As a result, the thinning neck and pending drop have a uniform director field and droplet snap-off is determined by the viscous properties of the liquid crystal as well as by thermal fluctuations of the interface. On the other hand, in sterically stabilized gibbsite where anchoring is strong, this ratio is significantly smaller than the neck thickness. In this case, the neck has an escaped radial director field and the neck thinning is retarded close to snap-off due to a topological energy barrier involved in the separation of the droplet from the thread.
New Journal of Physics 02/2012; 14(2):023010. DOI:10.1088/1367-2630/14/2/023010 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an experimental study on mixtures of the gibbsite platelets and silica nanospheres of the same charge sign. In these mixtures the spheres act as depletants changing the phase behaviour of the platelet system, for which two very different time scales can be distinguished. At short times as a result of a strong depletion attraction in the system a kinetically arrested phase was split off. Subsequently, on a time scale of months the top phase separated in an isotropic and a columnar phase on top of the arrested phase. We rationalize this gravity induced liquid crystal formation in terms of a platelet sedimentation and a strong depletion attraction. The sedimentation driven columnar phase is highly ordered as we show by microradian X-ray diffraction experiments
[Show abstract][Hide abstract] ABSTRACT: In this chapter we consider the depletion interaction between two flat plates and between two spherical colloidal particles
for different depletants (polymers, small colloidal spheres, rods and plates). First of all we focus on the depletion interaction
due to a somewhat hypothetical model depletant, the penetrable hard sphere (phs), to mimic a (ideal) polymer molecule.
Colloids and the Depletion Interaction, 05/2011: pages 57-108;
[Show abstract][Hide abstract] ABSTRACT: We present small angle x-ray scattering data of single-domain nematic and columnar liquid crystal phases in suspensions of sterically stabilized gibbsite platelets. The measurements are performed with different sample orientations to obtain information about the three-dimensional structure of the liquid crystalline phases. With the x-ray beam incident along the director of the nematic phase a strong correlation peak is observed corresponding to the side-to-side interparticle correlations, which suggests a columnar nematic structure. Upon sample rotation this side-to-side correlation peak of the nematic shifts to higher Q-values, suggesting the presence of strong fluctuations of small stacks of particles with different orientations, while the overall particle orientation is constant. In the hexagonal columnar phase, clear Bragg intercolumnar reflections are observed. Upon rotation, the Q-value of these reflections remains constant while their intensity monotonically decreases upon rotation. This indicates that the column orientation fluctuates together with the particle director in the columnar phase. This difference between the behaviour of the columnar and the nematic reflections upon sample rotation is used to assign the liquid crystal phase of a suspension consisting of larger platelets, where identification can be ambiguous due to resolution limitations.
[Show abstract][Hide abstract] ABSTRACT: We consider the birefringence patterns of nematic liquid crystals of gibbsite platelets at interfaces and in an aligning magnetic field. In solvents with a refractive index close to the particle refractive index, the intrinsic birefringence of the platelets dominates, resulting in positive birefringence, while if the solvent has a considerably different refractive index, birefringence due to the anisometric particle shape wins over the intrinsic birefringence, resulting in negative birefringence. The inversion of the birefringence leads to fascinating birefringent patterns.
[Show abstract][Hide abstract] ABSTRACT: We present an experimental study on the attractive glass formation in mixtures of colloidal gibbsite platelets and silica spheres. The platelets are 233 nm in diameter, and the diameter ratio between the platelets and the spheres is 13.8. The glass formation is induced by the short-ranged depletion attraction caused by the spheres. At sufficiently high sphere concentrations the depletion attraction is strong enough that the mixtures do not form the equilibrium liquid-crystalline phases, but the glass formation occurs instead. Using microradian X-ray diffraction measurements we provide detailed information about the structure of the arrested glass state. In mixtures with weaker depletion it consists of liquid-crystalline pockets embedded in the less ordered glassy surrounding. With a stronger attraction in the system the size of these domains decreases, until the structure of the attractive glass becomes completely disordered and isotropic.
[Show abstract][Hide abstract] ABSTRACT: We investigate the effect of a magnetic field on the shape and director field of nematic droplets in dispersions of sterically stabilized and charge-stabilized colloidal gibbsite platelets with a negative diamagnetic anisotropy. Depending on the magnetic field strength and tactoid size, we observe with polarized light microscopy several interesting structures, with different shapes and director fields both with and without defects. In particular, our findings provide the first experimental evidence for the existence of the split-core defect structure predicted ten years ago by Mkaddem and Gartland [Phys. Rev. E 62, 6694 (2000)]. The split-core structure is a metastable director-field configuration that can be stabilized by a sufficiently strong externally applied magnetic field but only if the diamagnetic anisotropy of the particles is negative. To account for our observations, we present a calculation of the stability regions of different shapes and director-field structures as a function of tactoid size, anchoring conditions, surface tension, elastic constants, and magnetic field strength. By fitting the experimental data to the theoretically predicted structures, we are able to extract values for the splay elastic constant, interfacial tension, and anchoring strength. Remarkably, we find significant differences between the two systems studied: for sterically stabilized gibbsite in bromotoluene the anchoring strength is one order of magnitude larger than that of aqueous gibbsite, with the latter exhibiting weak and the former strong anchoring of the director field to the interface. The splay elastic constants that we obtain are in agreement with earlier experiments, simulations, and theory, while the interfacial tension and anchoring strength are considerably larger than what was found in earlier experiments.
The Journal of Chemical Physics 01/2011; 134(4):044904. DOI:10.1063/1.3520389 · 2.95 Impact Factor