“Shake Gels” Elena Loizou 12 May 2006 1. Zebrowski, J.; Prasad, V.; Zhang, W.; Walker, L. M.; Weitz, D. A. “Shake-gels: shear-induced gelation of Laponite/PEO mixtures”, Colloids and Surfaces A: Physicochemical and Engineering Aspects 2003, 213, (2-3), 189-197. 2. Pozzo, D. C.; Walker, L. M. “Reversible shear gelation of polymer–clay dispersions”, 2004, 240, (1-3), 187-198. Elena Loizou 12 May 2006

What are the shake gels? Low viscosity fluids that when shaken form gels. Mixtures of a colloid and a polymer at specific range of concentrations Characteristics of shear-induced gels: Turbid, stiff, viscoelastic Can support their own weight when the jar is inverted When they left at rest they slowly relax back to a fluid “Half-cooled gelatin dessert”

Why shake gels are interesting? Why polymer - colloid dispersions are interesting? Can be used as: rheological modifiers – paints, cosmetics, food additives in coatings gas or solvent barriers Why shake gels are interesting? Have potential applications in industry: shock absorbers for cars transporters for materials (e.g. solids) drilling mud for petroleum extraction Such gels might be used in shock absorbers for cars, forming into a gel when the car drives over a hole. Solid particles would remain suspended in the gelled phase are they are transported, and then sediment as the gel relaxes at rest. Oil companies, often need to pump filler materials like sand into oil reservoirs, in order to continue the oil extraction Filtration and separation of colloidal particles may also benefit by the reversible gelation, as the filtration could be enhanced in the reversible gelation region.

Increasing PEO concentration Shake Gels - First observed : silica spheres (nm) + polyethylene oxide (PEO) solution gel shake gel Increasing PEO concentration “Shake gels” observed near the surface saturation limit Cabane, B.; Wong, K.; Lindner, P.; Lafuma, F. The society of Rheology 1997, 41, (3), 531-547.

Mechanism of shear-gelation: Occurs at a regime near the saturation of the particle surface with polymer At Rest: PEO chains weakly adsorbed onto particles form small aggregates Applied Shear: The small aggregates deform expose additional particle surface to the bulk new polymer segments adsorbed onto the fresh surface More polymer bridges between particles Cessation of Shear: thermal motions drive the polymer to desorb and obtain its original configuration, bridging is reduced  gels relax back to a fluid These bridges are rapidly form a network that spans the entire solution and produce a gels. The gels is stable while the shear is applied The reversibility is possible because only few polymer bridges are created and the polymer can easily desorb without large effort. The energy associate with adsorption is much less than that associated with the entropy of the polymer coil. Even those “shake gels” dispersions are stable and to not flocculate, the separation of the solids from the solvent can be achieved by rubbing the samples between the fingers

Discoid clay particles Clay : Laponite charged coin-like particles 25-30 nm in diameter 1 nm in thickness Crystal Structure Disc particle Stack of particles Discoid particles have flat surface and can show different structures and particle mobility than spherical particles of different size. Choose Laponite Laponite has extensively studied the last few years. The disc shape structure of Laponite, arise from its crystal structure. Where 6 octahedral magnesium ions sandwiched between two layers of 4 tetrahedral silicon atoms. These groups are balanced by 20 oxygen atoms and 4 hydroxyl groups. The height of the unit represents the thickness of the Laponite disc. The unit cells is repeated 30-40 thousands times in the two directions, to give a single disc. An ideal structure would have a neutral charge. In practice some magnesium ions are substituted by lithium and there are some spaces empty. So the empirical formula is this one: The negative charges in the dry form are balanced by sodium ions. A small positive charge originates from broken bonds along the rim. Crystals are arranged into stacks and held together electrostatically by sharing sodium ions in the interlayer region between adjacent crystals. Na0.7+ [(Si8Mg 5.5 Li0.3) O20(OH)4]-0.7

Laponite: Dispersion / Exfoliation During dispersion of Laponite into water, the sodium ions are released form the surface of the particle and form an electrical double layer around the Laponite disks and result in a negative surface charge for the disc. That causes the particles to repel each other and finally exfoliated. Aqueous pure Laponite solutions were observed to form gels with concentration above 3wt%. Exfoliation platelets separate from each other

Mechanism of gelation - Still a considerable debate Attractive interactions Repulsive interactions OR The mechanism of gelation is still a considerable debate in the literature. Some authors claim that the gel formation is because of attractive electrostatic interactions between the edge and the faces of the particles. That form “ house of cards structure”. This theory was proven only by simulations , and no experimental evidence exist. Scattering and microscopy experiments, suggest an alternative mechanism, where repulsive interactions dominate. This repulsion arise from the overlapping double layer. In reality both mechanism could be valid in different regions, based on the C, ionic strength and pH. It could be a transition from a repulsive gel to an attractive gel Electrostatic Coulomb Repulsion Van der Waals Attraction Tanaka, H.; Meunier, J.; Bonn, D. Physical Review E 2004, 69, 031404

Poly(ethylene oxide) - PEO Water-soluble, synthetic polymer Simple basic unit : (-CH2CH2O-)n When dissolves in water, is characterized Adsorbs onto Laponite platelets Hydrophilic interactions through O Hydrophobic interactions through CH2CH2

Phase Diagram of Laponite-PEO PEO : Mw = 300 000 g/mol Shear Thickening samples Shake Gel samples Liquid samples To explore the phases observed, they vary the concentration of PEO and Laponite in the mixture. With in the limit of the concentrations studied, they observed 3 phases: Shake gel phase (triangles), gels upon shaking, increase viscosity, elasticity, opacity Shear thickening phase (circles), increase in viscosity, but no elasticity, can still flow (more turbid) Liquid phase, with vigorous shaking (filled squares) upon shaking no visible gelation or shear thickening. (foam was observed on the surface of the sample, suggesting free PEO in solution. (PEO is surface active) Zebrowski, J.; Prasad, V.; Zhang, W.; Walker, L. M.; Weitz, D. A. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2003, 213, (2-3), 189-197.

Phase Diagram of Laponite-PEO The phase diagram, is reproduced and expanded. The figure shows the observed phases as a function of clay concentration in weight percent. Instead of polymer concentration they plot it as a function of the mixture concentration, where is the total mass of polymer/ total area of clay surface. To show that it depends on how much polymer covers the clay surface. At low PEO concentrations, most of the chains are adsorbed, and bringing can occur oven a very small fraction of the particle population. Most of the particles are individual particles and the sample behaves like pure clay solution of similar concentration. This is a stable, low viscosity solution. At higher PEO concentration, but below the surface saturation there is enough polymer to bridge between particles, and cause flocculation. Draw particles together and form flocs, which are permanent turbid gels. Show no relaxation with time As PEO concentration increase a lot, above the clay saturation, the samples become transparent and fluid. Upon agitation a foam layer develops which indicates the presence of free polymer chain in solution. Called: “foaming stable solution” The clay is completely saturate and the polymer layer prevents the discs from aggregating. At concentration near the surface saturation value, the size of shake gels were observed. There is little free clay surface and the aggregates are smaller. Shear can deform those aggregates and expose fresh surface to the solution, create more bridging and more aggregation. Interesting is that the shear induced gelation cannot be induced when the samples are subject to simple shear in a couette cell. The shear gelation can in the Laponite-PEO system can occur only upon agitation. This is probably because shear can orient the clay aggregates and then the number of effective collision is reducing, prevent further aggregation. But simple shear field can sustained the gel phase that was grated upon agitation. Pozzo, D. C.; Walker, L. M.,Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004, 240, (1-3), 187-198.

Characterization Techniques Scattering -light -neutron -x-ray Rheology -flow -oscillatory Microscopy -SEM -TEM -AFM Birefringence

Light Scattering Dynamic light scattering (DLS) Relies on time-dependent fluctuations on the intensity due to Brownian motions of molecules Measure the diffusion coefficient of the molecules Determine a hydrodynamic radius, Rh The size range: 1 nm - 500 nm Second order autocorrelation function Experimental parameters Decay rate Decay time Diffusion coefficient

Small Angle Neutron Scattering (SANS) Characteristic dimension Spacing between particles λ: neutron wavelength, θ: scattering angle, Q: scattering vector http://www.ncnr.nist.gov/summerschool/information/SANS_tutorial.pdf

Contrast Matched SLD = 6.4 x1010 cm-2 Laponite SLD = 4.2x1010 cm-2 (69% D2O) PEO SLD = 0.6x1010 cm-2 (17% D2O) SLD = 6.4 x1010 cm-2 SLD = -0.6 x1010 cm-2 Nelson, Andrew, Neutron and Light Scattering Studies of Polymers Adsorbed on Laponite. University of Bristol, 2002 Pynn, Roger, Neutron Scattering - A PRIMER. Los Alamos Neutron Science Center (LANSCE), 1990

Phase Diagram of Laponite-PEO The phase diagram, is reproduced and expanded. The figure shows the observed phases as a function of clay concentration in weight percent. Instead of polymer concentration they plot it as a function of the mixture concentration, where is the total mass of polymer/ total area of clay surface. To show that it depends on how much polymer covers the clay surface. At low PEO concentrations, most of the chains are adsorbed, and bringing can occur oven a very small fraction of the particle population. Most of the particles are individual particles and the sample behaves like pure clay solution of similar concentration. This is a stable, low viscosity solution. At higher PEO concentration, but below the surface saturation there is enough polymer to bridge between particles, and cause flocculation. Draw particles together and form flocs, which are permanent turbid gels. Show no relaxation with time As PEO concentration increase a lot, above the clay saturation, the samples become transparent and fluid. Upon agitation a foam layer develops which indicates the presence of free polymer chain in solution. Called: “foaming stable solution” The clay is completely saturate and the polymer layer prevents the discs from aggregating. At concentration near the surface saturation value, the size of shake gels were observed. There is little free clay surface and the aggregates are smaller. Shear can deform those aggregates and expose fresh surface to the solution, create more bridging and more aggregation. Interesting is that the shear induced gelation cannot be induced when the samples are subject to simple shear in a couette cell. The shear gelation can in the Laponite-PEO system can occur only upon agitation. This is probably because shear can orient the clay aggregates and then the number of effective collision is reducing, prevent further aggregation. But simple shear field can sustained the gel phase that was grated upon agitation.

Layer thickness and absorbed amount Polymer Layer Thickness: 2-3 nm On each face: 1-1.5 nm Absorbed amount : 0.6-0.9 mg/m2 Core-Shell Model They added polymer to the clay and observed that the polymer either prevents gelation or slows it down extremely, based on the molecular weight of the polymer and the clay and polymer concentration. They performed SANS experiments and observed that the scattering intensity of the mixture is not just the addition of the intensities of the bare laponite and the free polymer. This means that there is some interaction between the polymer and the laponite. Weak molecular forces, such as Van der Waals forces, provide the driving force for physical adsorption Gamma A: interfacial amount of polymer Gamma P: assumes that all chains are absorbed and calculate interfacial amount of polymer Did not find any dependence o Lal, J.; Auvray, L. “Interaction of polymer with clays”, Journal of Applied Crystallography 2000, 33, (1), 673-676. Lal, J.; Auvray, L. “Interaction of polymer with discotic clay particles”, Molecular Crystals and Liquid Crystals 2001, 356, 503-515.

Absorbed amount and layer thickness Edge thickness: 1.5 - 4.5 nm Face thickness: 1.5 nm Core-Shell Model The shell is extended to the sides of the clay Absorbed amount : 0.7mg/m2 Nelson, A.; Cosgrove, T. “A Small-Angle Neutron Scattering Study of Adsorbed Poly(ethylene oxide) on Laponite”, Langmuir 2004, 20, (6), 2298-2304.

Phase Diagram of Laponite-PEO The phase diagram, is reproduced and expanded. The figure shows the observed phases as a function of clay concentration in weight percent. Instead of polymer concentration they plot it as a function of the mixture concentration, where is the total mass of polymer/ total area of clay surface. To show that it depends on how much polymer covers the clay surface. At low PEO concentrations, most of the chains are adsorbed, and bringing can occur oven a very small fraction of the particle population. Most of the particles are individual particles and the sample behaves like pure clay solution of similar concentration. This is a stable, low viscosity solution. At higher PEO concentration, but below the surface saturation there is enough polymer to bridge between particles, and cause flocculation. Draw particles together and form flocs, which are permanent turbid gels. Show no relaxation with time As PEO concentration increase a lot, above the clay saturation, the samples become transparent and fluid. Upon agitation a foam layer develops which indicates the presence of free polymer chain in solution. Called: “foaming stable solution” The clay is completely saturate and the polymer layer prevents the discs from aggregating. At concentration near the surface saturation value, the size of shake gels were observed. There is little free clay surface and the aggregates are smaller. Shear can deform those aggregates and expose fresh surface to the solution, create more bridging and more aggregation. Interesting is that the shear induced gelation cannot be induced when the samples are subject to simple shear in a couette cell. The shear gelation can in the Laponite-PEO system can occur only upon agitation. This is probably because shear can orient the clay aggregates and then the number of effective collision is reducing, prevent further aggregation. But simple shear field can sustained the gel phase that was grated upon agitation.

Dynamic Light Scattering Laponite:1.25 wt % Dimensionless time constant Fixed the Laponite Concentration and vary the PEO concentration, Measure the decay time from field-field autocorrelation function. The larger the scatter, the more slowly will diffuse, the larger the decay time. At low PEO concentration the PEO bridges several clay particles, t=2, means that aggregation and bridging At higher PEO concentration the PEO completely coats the laponite discs, absorbs flat onto the disc without increasing much the size of the disc and particles decay like pure laponite particles. Decay time of Laponite-PEO mixture Decay time of Pure Laponite (0.24 ms)

Relaxation after shear-induced gelation 1.5% (w/w) Laponite – 0.45% (w/w) PEO 10 C 15 C 30 C 25 C 20 C G*: complex modulus G’ : elastic modulus G’’ : viscous modulus Sample was manually shaken and loated into the shear cell where was sheared for 5 min at a rate of 300 inverse seconds and then let to relax to characterize the relaxation. The relaxation of the magnitude of the complex modulus vs. time The relaxation is very sensitive to the sample temperature, As the temperature decrease the, relaxation slows And at 10 C is incomplete even after 12h. This observation is consistent with the explanation that relaxation occurs because of thermal motions.

Arrhenius plot T : absolute temperature (K) A : non thermal constant : characteristic relaxation time T : absolute temperature (K) A : non thermal constant EA : activation energy (eV) KB : Boltzman’s constant (8.61738 x 10-5 eV/K) Plot the ln of characteristic relaxation time vs. Temperature. Suggests that the breakup of the aggregates depends on an activated progress. The temperature behavior arises from the existence of an energetic barrier for the desorption of the unstable attach polymer segments. Activation Energy (EA) = 107 kJ/mol

Aging Effects T=25 C 1.5% (w/w) Laponite – 0.45% (w/w) PEO 21 day old An aging effect is observed on the solution over time. Samples more than 1 week old, when shaken still create gels, but those gels do not completely relax back to a liquid. The relaxation slows with time and is incomplete. The 3-weeks old sample, shows the elastic modulus instead of decreasing is increasing with time and the loss modulus remain constant. These gels have greater mechanical strength than the shear induced gel phase. 1 day old

Scattering Profiles - pure solutions 1.5% (w/w) Laponite Thin particle Form factor of Non-interacting thin discs R = 13.3nm H = 0.8 nm 0.45% (w/w) PEO Random coils with Excluded Volume Interactions

Scattering Profiles Slope: -1 Elongated objects Slope: -2 Thin Disc 1.5 % (w/w) Laponite – 0.45% (w/w) PEO - (D2O) Slope: -1 Elongated objects 25 C Slope: -2 Thin Disc 10 C Gelled phases are the same So T, does not affect the structure At T=10 C the relaxation is incomplete and thermal fluctuations not strong enough to break up the aggregates Data are collected when the gel is sheared at 300 s-1 to prevent the sample from relaxing (open squares) And at rest after 2h of relaxation. (open circles) The scattering intensity of the mixture is larger than the sum of the 2 components individually, indicating some type of interaction. The relaxed scattering profile, shows a shoulder same as the pure laponite solution. The shoulder though is shifted to lower q values indicating a little bit bigger particles, due to the present of the PEO. Samples show no anisotropy, but are highly birefringent and show elogational elasticity (pull strong fiber) The birefringence is likely due to the refractive index differenced, between the large elongated aggregates, that shear oriented and the surrounding water. The slope of -2 , thin discs, the slope of -1, for elongated 1-D objects. –consistence with birefringence data Experiment was repeated at 10C . Scattering from the gelled and relax phase are the same. And also the Gelled phases at 25 and 10 C are the same. So the T does not affect the structure of the gel, So the relaxation must be a result of break up of aggregates due to thermal fluctuation. At 10 C the relaxation is incomplete, because the thermal fluctuations are not strong enough to break up all the aggregates. The sample is kinetically trapped in a metastable state.

Phase Diagram of Laponite-PEO

Contrast Matched the Clay 1.5% (w/w) Laponite + PEO (69% D2O) Highly stretched PEO Shake Gel Flat adsorbed 2-D structure 25 C Permanent Gel Medium PEO Low PEO 10 C Foaming solution Contrast matched the particle, the clay is invisible to the scattering and we see only polymer-polymer correlations. The scattering is from the adsorbed polymer chains and from the free polymer. The slope of -2 is consistent with 2-D object. That means the PEO is flatty absorbed on the clay and is consisted of trains and small loops. The scattering from the relax state is similar to pure clay. But the sloulder moves to lower q-values that means bigger particles, due to the PEO. Low PEO content. Gelatin appearance, and no change is properties, after agitation. The slope is -1.6, caused by the highly streched PEO bridges that form macroscopic aggregates. As the polymer is streched the shape is changed dorm a flat adsorbed 2-D structure, to a conformation that is close to extended chains. Excess PEO, profiles are almost identical and similar to pure clay. PEO coats the clay and adopts its shape. The surface is saturated with PEO and aggregation is suppressed by steric stabilization. Shearing shows no anisotropy. Medium PEO High PEO PEO coats the clay and adopts its shape

Contrast Matched the PEO 1.5% (w/w) Laponite – 0.45% (w/w) PEO - (17 % D2O) 25 C Shake Gel The scattering differences are smaller The polymer is the one that experience the large deformational changes upon shear The polymer is invisible and we the contrast is between the clay particles and the solvent. The sample show a small change is scattering profile, while sheared and after relaxation. The decrease in the Rg, after relaxation shows that shear aggregation brings the clay particle closer together. The scattering differences are smaller than that found in the experiments with pure D2O and with 69% D20. Therefore the polymer dominated the small q-scattering and experience largest deformational changes on aggregation.

Conclusions The phase diagram, is reproduced and expanded. The figure shows the observed phases as a function of clay concentration in weight percent. Instead of polymer concentration they plot it as a function of the mixture concentration, where is the total mass of polymer/ total area of clay surface. To show that it depends on how much polymer covers the clay surface. At low PEO concentrations, most of the chains are adsorbed, and bringing can occur oven a very small fraction of the particle population. Most of the particles are individual particles and the sample behaves like pure clay solution of similar concentration. This is a stable, low viscosity solution. At higher PEO concentration, but below the surface saturation there is enough polymer to bridge between particles, and cause flocculation. Draw particles together and form flocs, which are permanent turbid gels. Show no relaxation with time As PEO concentration increase a lot, above the clay saturation, the samples become transparent and fluid. Upon agitation a foam layer develops which indicates the presence of free polymer chain in solution. Called: “foaming stable solution” The clay is completely saturate and the polymer layer prevents the discs from aggregating. At concentration near the surface saturation value, the size of shake gels were observed. There is little free clay surface and the aggregates are smaller. Shear can deform those aggregates and expose fresh surface to the solution, create more bridging and more aggregation. Interesting is that the shear induced gelation cannot be induced when the samples are subject to simple shear in a couette cell. The shear gelation can in the Laponite-PEO system can occur only upon agitation. This is probably because shear can orient the clay aggregates and then the number of effective collision is reducing, prevent further aggregation. But simple shear field can sustained the gel phase that was grated upon agitation.

Conclusions YES !!! Shake gels were observed with discoid Laponite particles when they were mixed with PEO Occur at a regime near saturation of clay surface with polymer Under shear  formation new polymer-clay bridges With cessation of shear  slowly relaxation due to thermal motions Relaxation depends on: -temperature -aging of the sample

Questions?

Structure Factor: gives information about the correlations of atomic position, and it can be measured only in concentrate systems. Form factor: corresponds to the particle shape. In dilute suspensions were the intensity depends only to the form factor, information about the particle size and shape can be obtained. The form factor is a Fourier transformation of the particle pair correlation function.