Roberto Piazza

Thermophoresis in colloidal suspensions

Thermophoresis is particle motion induced by thermal gradients. Akin to other driven transport processes, such as the Soret effect in simple fluid mixtures, or electrophoresis and diffusiophoresis in colloidal suspensions, it is, both experimentally and theoretically, a challenging subject. Rather than being a comprehensive recollection, this review aims to be a critical re-examination of the experimental and theoretical tools used to investigate thermophoresis, and of some recent relevant results that may unravel novel aspects of colloid solvation forces. The perspectives of thermophoresis as a tool for particle manipulation in microfluidics are also emphasized.

The Classical Nature of Thermal Conduction in Nanofluids

We show that a large set of nanofluid thermal conductivity data falls within the upper and lower Maxwell bounds for homogeneous systems. This indicates that the thermal conductivity of nanofluids is largely dependent on whether the nanoparticles stay dispersed in the base fluid, form large aggregates, or assume a percolating,fractal confguration. The experimental data, which are strikingly analogous to those in most solid composites and liquid mixtures, provide strong evidence for the classical nature of thermal conduction in nanofluids.

Interactions and phase transitions in protein solutions

Understanding interactions and phase transitions in protein solutions is essential in order to develop systematic protein crystallisation strategies. Recent studies show that proteins near crystallisation behave as particles interacting via a very short-range attractive potential. The microscopic origin of this attractive term, and its strong dependence on the nature of the crystallisation agent, is, however, still obscure. The interplay of crystal nucleation with formation of weakly bonded amorphous aggregates also deserves further analysis.

Thermophoresis: moving particles with thermal gradients

Thermophoresis is particle motion induced by thermal gradients. Akin to other nonequilibrium transport processes such as thermal diffusion in fluid mixtures, it is both experimentally and theoretically a challenging subject. New insights stemming from careful experimental surveys and strict theoretical models have however shed light on the underlying physical mechanisms, enabling depiction of thermophoresis as a subtle interfacial effect. These recent advancements open up alluring perspectives to exploit thermophoresis as a novel tool in macromolecular fractionation, microfluidic manipulation, and selective tuning of colloidal structures.

Critical properties of nonionic micellar solutions

Solutions of the nonionic amphiphile n‐dodecyl octaoxyethylene glycol monoether (C12E8) in D2O and in salty H2O have been investigated by static and dynamic light scattering. It is found that the micellar properties are little influenced by the choice of the solvent, whereas the critical properties (in particular, the critical exponents γ and ν) show a marked solvent effect. The interpretation of the observed critical behavior in terms of first‐order correction‐to‐scaling contributions is discussed.

Thermophoresis and Thermoelectricity in Surfactant Solutions

In electrolyte solutions, the differential migration of the ionic species induced by the presence of a thermal gradient leads to the buildup of a steady-state electric field. Similarly to what happens for the Seebeck effect in solids, the sample behaves therefore as a thermocell. Here, we provide clear evidence for the presence of thermoelectric fields in liquids by detecting and quantifying their strong effects on colloid thermophoresis. Specifically, by contrasting the effects of the addition of NaCl or NaOH on the Soret effect of micellar solutions of sodium dodecyl sulfate, we show that the presence of highly thermally responsive ions such as OH(-) may easily lead to the reversal of particle motion. Our experimental results can be quantitatively explained by a simple model that takes into account interparticle interactions and explicitly includes the micellar electrophoretic transport driven by such a thermally generated electric field. The chance of carefully controlling colloid thermophoresis by tuning the solvent electrolyte composition may prove to be very useful in microfluidic applications and field-flow fractionation methods.

Thermal-lensing measurement of particle thermophoresis in aqueous dispersions

We show that thermophoresis (particle drift driven by thermal gradients) in aqueous solutions can be measured by using an all-optical thermal-lensing setup, where a temperature gradient is set by a near-infrared laser beam with no need of light-absorbing dyes. After discussing the principles of the method, we study by numerical simulation the nature and extent of parasitic thermal-convection effects, and we describe an optical setup designed to limit them. We finally present preliminary results on thermophoresis in micellar solutions and colloidal dispersions.

Static and dynamic light scattering study of fluorinated polymer colloids with a crystalline internal structure

Abstract The light scattering properties of polymer colloids made of polytetrafluoroethylene with added copolymers are described in some detail. The particles possess a partially crystalline internal structure which makes them optically anisotropic, and consequently gives rise to a strong depolarized component in the scattered light intensity. We present a complete theory of static and dynamic light scattering which includes the effects of polycrystallinity, of optical polydispersity, and of interparticle interactions. We also discuss applications to surfactant adsorption studies, to sedimentation experiments and to fractal aggregation processes

Interactions in protein solutions near crystallisation : a colloid physics approach

Abstract I present a review of current ideas from colloid physics which might be relevant in order to understand the onset of crystallisation and amorphous aggregation processes in protein solution. In particular, the inadequacy of DLVO theory to account for all phenomenological aspects of crystallisation, such as salt-specificity, and for the basic features of the phase diagram, such as the presence of a metastable fluid–fluid separation, is discussed. The fundamental role of additional short-range attractive forces, microscopically orginating from the salting-out effect, is conversely stressed. In order to establish a simple model of protein interparticle interactions near crystallisation, I discuss some recent results obtained by our group for the osmotic compressibility of the metastable fluid phase of hen egg-white lysozyme. Light scattering measurements were performed in an extended volume fraction range at pH=4.7 as a function of temperature, adding NaC1 to screen the electrostatic interactions. The experimental compressibility up to particle volume fractions Φ≈0.23 is very successfully compared to the theoretical expression for a model of adhesive (“sticky”) hard spheres. This surprising quantitative agreement, obtained using a very simple form for the effective interparticle force, suggests that the thermodynamics of the system in the fluid phase is mainly determined by the very short-range nature of the interactions, and is rather insensitive to the detailed form of the potential.

Optical measurements of the thermal properties of nanofluids

The authors show that the thermal conductivity and diffusivity of colloidal particle dispersions can be rapidly obtained with high accuracy and reproducibility by exploiting a noninvasive, all-optical thermal lensing method. Applications of this technique to model suspensions of spherical monodisperse particles suggest that classical models for the effective properties of composite media hold up to rather high volume fractions, while no “anomalous” thermal conductivity effects are found.