John Y. Walz

Freeze casting of porous materials: review of critical factors in microstructure evolution

Abstract Freeze casting is a promising technique to fabricate porous materials with complex pore shapes and component geometries. This review is aimed to elaborate the fundamental principles of the porous microstructure evolution and critical factors that influence the fundamental physics involved in freeze casting of particulate suspensions. The discussion separately analyses homogeneous and directional freeze casting for both aqueous and non-aqueous systems. The effects of additives, freezing conditions, suspension solids loading and particle size on pore shape, size and morphology evolution are discussed. Special techniques based on modified freeze casting, such as freeze tape casting, double sided freeze casting and field directed freeze casting, are also included.

Interaction forces between colloidal particles in a solution of like-charged, adsorbing nanoparticles.

We have measured the force between a weakly charged micron-sized colloidal particle and flat substrate in the presence of highly charged nanoparticles of the same sign under solution conditions such that the nanoparticles physically adsorb to the colloidal particle and substrate. The objective was to investigate the net effect on the force profile between the microparticle and flat substrate arising from both nanoparticle adsorption and nanoparticles in solution. The experiments used colloidal probe atomic force microscopy (CP-AFM) to measure the force profile between a relatively large (5 μm) colloidal probe glass particle and a planar glass substrate in aqueous solutions at varying concentrations of spherical nanoparticles. At very low nanoparticle concentrations, the primary effect was an increase in the electrostatic repulsion between the surfaces due to adsorption of the more highly charged nanoparticles. As the nanoparticle concentration is increased, a depletion attraction formed, followed by longer-range structural forces at the highest nanoparticle concentrations studied. These results suggest that, depending on their concentration, such nanoparticles can either stabilize a dispersion of weakly-charged colloidal particles or induce flocculation. This behavior is qualitatively different from that in nonadsorbing systems, where the initial effect is the development of an attractive depletion force.

Influence of Charged Nanoparticles on Colloidal Forces: A Molecular Simulation Study

We employ the grand canonical Monte Carlo simulation technique to investigate the influence of charged nanoparticles (macro-ions) on the force between colloidal objects. Specifically, the structure and osmotic pressure of a system of screened Coulomb (Yukawa) particles confined between charged planar walls are simulated. We observe osmotic pressure to oscillate with wall separation and these oscillations to correspond to changes in the number of nanoparticle layers present in the slit pore. Using the Derjaguin approximation, we estimate the overall force between a colloidal sphere and a flat surface and compare our predictions to recent atomic force microscopy (AFM) results (Tulpar, A.; Van Tassel, P. R.; Walz, J. Y. Langmuir 2006, 22, 2876-2883). In excellent agreement with experiment, we find the wavelength of the force versus distance oscillations to scale as c(nu), with c being the bulk nanoparticle concentration and nu = -0.31 +/- 0.01; that is, slightly lower in magnitude from the expected value -1/3 based on average molecule spacing. By considering an order parameter measuring the extent to which neighboring particles form hexagonal symmetry, we show structural order within confined nanoparticle systems to be significantly enhanced as compared to that of bulk systems, despite being quite insensitive to wall separation. Wavelength scaling and order parameter analysis together suggest the confined macro-ion systems to be somewhat glasslike.

Stabilization of weakly charged microparticles using highly charged nanoparticles.

An experimental study was performed to understand the ability of highly charged nanoparticles to stabilize a dispersion of weakly charged microspheres. The experiments involved adding either anionic (sulfate) or cationic (amidine) latex nanoparticles to dispersions of micrometer-sized silica particles near the silica isoelectric point (IEP). Although both types of nanoparticles increased the zeta potential of the silica microspheres above the value at which dispersions containing only silica spheres remained stable, only with the amidine nanoparticles was stability obtained. Adsorption tests with flat silica slides showed that the amidine nanoparticles deposited in much greater numbers onto the silica, producing multilayer coverage with adsorbed particle densities that were roughly three times that obtained with the sulfate nanoparticles. A model calculating the DLVO interaction between the silica spheres in which the adsorbed nanoparticle layers were treated as a continuous film with dielectric properties between those of polystyrene and water predicted stability for both systems. It is hypothesized that the relatively low adsorption of the sulfate nanoparticles (fractional surface coverages ≤ 25%) led to patches of bare silica on the microspheres that could align during interaction due to Brownian motion. These results indicate that highly charged nanoparticles can be effective stabilizers provided the level of adsorption is sufficiently high. It was also found that the zeta potential alone is not a sufficient parameter for predicting stability of these binary systems.

Simultaneous investigation of sedimentation and diffusion of a single colloidal particle near an interface.

We describe here a new procedure for the simultaneous investigation of sedimentation and diffusion of a colloidal particle in close proximity to a solid, planar wall. The measurements were made using the optical technique of total internal reflection microscopy, coupled with optical radiation pressure, for dimensionless separation distances (gap width/radius of particle) ranging from 0.01 to 0.05. In this region, the hydrodynamic mobility and diffusion coefficient are substantially reduced below bulk values. The procedure involved measuring the mean and the variance of vertical displacements of a Brownian particle settling under gravity toward the plate. The spatially varying diffusion coefficient was calculated from the displacements at small times (where diffusive motion was dominant). The mobility relationship for motion normal to a flat plate was tested by measuring the average distance of travel versus time as the particle settled under the constant force of gravity. For the simple Newtonian fluid used here (aqueous salt solution), the magnitude of the diffusion coefficient and mobility, plus their dependence on separation distance, showed excellent agreement with predictions. This new technique could be of great value in measuring the mobility and diffusion coefficient for near-contact motion in more complex fluids for which the hydrodynamic correction factors are not known a priori, such as shear-thinning fluids.

Correlations between the thermal vibrations of two cantilevers: Validation of deterministic analysis via the fluctuation-dissipation theorem

We validate a theoretical approach for analyzing correlations in the fluctuations of two cantilevers in terms of a deterministic model, using the fluctuation-dissipation theorem [M. R. Paul and M. C. Cross, Phys. Rev. Lett. 92, 235501 (2004)]. The validation has been made possible through measurement of the correlations between the thermally stimulated vibrations of two closely spaced micrometer-scale cantilevers in fluid. Validation of the theory enables development of a method for characterizing fluids, which we call correlation force spectrometry.

Depletion flocculation induced by synergistic effects of nanoparticles and polymers.

The depletion flocculation of stable suspensions of charged microparticles (1.2 μm diameter polystyrene) by mixtures of silica nanoparticles (7 nm) and poly(acrylic) acid (PAA, Mn = 24 500) was studied. The experiments revealed a very clear critical flocculation concentration of PAA that was lowered by the addition of the silica nanoparticles. Equilibrium pair potentials between the particles, calculated from force profiles measured between a silica particle and plate using colloid probe atomic force microscopy, showed clear a synergistic effect in that the magnitude of the depletion and structural forces produced in the mixed PAA/nanoparticle system was significantly greater than the sum of the forces in systems containing only PAA or only nanoparticles. This effect arises from adsorption of the PAA onto the nanoparticles, creating much larger charged complexes. Comparing the calculated pair potentials to the observed flocculation results revealed that flocculation occurred once the magnitude of the secondary energy well formed by the attractive depletion force exceeded 5 kT.

Fabrication of Porous Nanocomposites with Controllable Specific Surface Area and Strength via Suspension Infiltration

Porous ceramics are promising candidates for a variety of applications, including separation membranes, catalyst supports, tissue engineering scaffolds, energy storage devices, and microelectronics. We describe a novel method for creating porous ceramics with controllable specific surface area and high strength. The fabrication procedure involves infiltrating aqueous suspensions of silica nanoparticles into a porous ceramic scaffold. The samples are then freeze-dried to maintain a homogeneous distribution of nanoparticles, followed by partial sintering to bond the infiltrated nanoparticles into place. By repeating this infiltration process multiple times, the specific surface area of the composite can be varied from less than one to well over 100 m(2)/g. It is also found that this infiltration increases the mechanical strength of the composite. Water flux experiments demonstrate the potential use of these materials as liquid membranes, with no detectable damage to the structure observed after these flux tests. While this initial work focused on silica nanoparticles and ceramic scaffolds, the basic approach would to applicable to a wide variety of other materials, meaning that the method described here would be generally applicable for creating porous materials with precisely controllable properties.

Synergistic effects of nanoparticles and polymers on depletion and structural interactions.

An experimental study was performed to investigate the synergistic effects of two different solution components on the depletion and structural forces between colloidal particles. Using silica nanoparticles and anionic poly(acrylic acid) polymer, it was found that the depletion and structural forces measured between a 30 μm diameter silica sphere and a flat silica plate (obtained using colloidal probe atomic force microscopy) were substantially greater than the sum of the forces obtained in systems containing only nanoparticles and only polymer. This result arises because the anionic polymer chains adsorb to the nanoparticles, creating a complex that is over twice as large as either component. Although the number density of depletants decreases with such complexation, the larger size results in much greater forces at longer ranges. In addition, the measured force profiles could be well described using a force model in which all components were treated as hard, charged spheres. The results clearly indicate that predicting the depletion force in systems with multiple depletant components, such as the one used here, can be much more complicated than simply adding the forces contributed from each component independently.

Rheology of fluids measured by correlation force spectroscopy

We describe a method, correlation force spectrometry (CFS), which characterizes fluids through measurement of the correlations between the thermally stimulated vibrations of two closely spaced micrometer-scale cantilevers in fluid. We discuss a major application: measurement of the rheological properties of fluids at high frequency and high spatial resolution. Use of CFS as a rheometer is validated by comparison between experimental data and finite element modeling of the deterministic ring-down of cantilevers using the known viscosity of fluids. The data can also be accurately fitted using a harmonic oscillator model, which can be used for rapid rheometric measurements after calibration. The method is non-invasive, uses a very small amount of fluid, and has no actively moving parts. It can also be used to analyze the rheology of complex fluids. We use CFS to show that (non-Newtonian) aqueous polyethylene oxide solution can be modeled approximately by incorporating an elastic spring between the cantilevers.