Claudio L. A. Berli

Electrokinetic flow of non-Newtonian fluids in microchannels.

A theoretical description of the electrokinetic flow of non-Newtonian fluids through slit and cylindrical microchannels is presented. Calculations are based on constitutive models of the fluid viscosity, and take into account wall depletion effects of colloids and polymer solutions. The resulting equations allow one to predict the flow rate and electric current as functions of the simultaneously applied electric potential and pressure gradients. It is found that (i) nonlinear effects induced by the shear-dependent viscosity are limited to the pressure-driven component of the flow, and (ii) the reciprocity between electroosmosis and streaming current is complied. Thus a generalized form of the force-flux relations is proposed, which is of interest in microfluidic applications.

The EOF of polymer solutions

The EOF of polymer solutions is analysed in the framework of continuum fluid mechanics and the standard electrokinetic model. Two key aspects are taken into consideration: the non‐Newtonian character of the fluid and the polymer concentration near the interface, which greatly modify the fluid viscosity in the region where electroosmosis takes place. A satisfactory mathematical model is derived for the electroosmotic mobility of solutions that present polymer depletion at the wall. The case of solutions containing polymers that adsorb onto the wall is briefly reviewed, and a preliminary approach is discussed for the limit of strong polymer adsorption. In order to illustrate the theoretical discussions, experimental data obtained from aqueous solutions of carboxymethyl cellulose in fused‐silica capillaries are presented. Relevant results are observed, which are appropriately captured by the modelling proposed. The fundamental phenomena discussed in this work are of interest in microfluidics and electrophoresis.

Nonlinear viscoelasticity and two-step yielding in magnetorheology: A colloidal gel approach to understand the effect of particle concentration

The yielding behavior of conventional magnetorheological (MR) fluids is revisited for a wide range of magnetic fields and particle concentrations under a colloidal gel perspective. A two-step yielding behavior is found at intermediate magnetic fields (∼10 kA/m) that can be explained as a transition from a strong-link to a weak-link (or transition) regime upon increasing the particle concentration in the MR fluid. This two-step yielding behavior is reminiscent of the classical concepts of static (frictional) and dynamic (Bingham) yield stress. By relating macroscopic elastic properties to a scaling fractal model, we could identify the prevalent gelation regime in MR fluids.

Connection between rheological parameters and colloidal interactions of a soy protein suspension

Abstract The viscosity of a 9.1% (w/w) aqueous suspension of soy proteins at pH 7.5, which constitutes a colloidal dispersion of negatively charged and hydrated particles, is analyzed in relation to colloidal interactions. An experimental study is carried out through the classical shear rate rheometry, in the range 20–50°C. Viscosity data at different temperatures and shear rates are superimposed onto a master plot, which shows the characteristic shear-thinning behavior of colloidal suspensions. A microstructural model for the low shear limit viscosity is derived to establish a quantitative relationship between the viscosity and the interparticle forces in the suspension. Apart from the electrical forces, the hydration repulsive force must be considered in the calculation of the protein–protein interaction energy, in order to achieve a satisfactory prediction of the high viscosity values obtained experimentally. It is also observed that the relaxation time associated to small shear deformations differs from the characteristic relaxation time of the steady shear flow. The theoretical analysis carried out in this work explains the effect of colloidal interactions on rheological parameters of the soy protein suspension.

Capillary Filling in Nanostructured Porous Silicon

An experimental study on the capillary filling of nanoporous silicon with different fluids is presented. Thin nanoporous membranes were obtained by electrochemical anodization, and the filling dynamics was measured by laser interferometry, taking advantage of the optical properties of the system, related with the small pore radius in comparison to light wavelength. This optical technique is relatively simple to implement and yields highly reproducible data. A fluid dynamic model for the filling process is also proposed including the main characteristics of the porous matrix (tortuosity, average hydraulic radius). The model was tested for different ambient pressures, porous layer morphology, and fluid properties. It was found that the model reproduces well the experimental data according to the different conditions. The predicted pore radii quantitatively agree with the image information from scanning electron microscopy. This technique can be readily used as nanofluidic sensor to determine fluid properties such as viscosity and surface tension of a small sample of liquid. Besides, the whole method can be suitable to characterize a porous matrix.

A structural viscosity model for magnetorheology

A structural viscosity model is proposed, which describes the shear viscosity from the balance between build up (magnetic field-induced clustering) and breakdown (shear-induced breakup) of particle aggregates. The model accounts for typical deviations from Bingham model predictions that are extensively reported in the MR literature. More precisely, the model (i) provides a physical ground for the observed Casson-like shear flow behaviour, (ii) predicts the existence of a low shear plateau in weak MR fluids, and (iii) asymptotically recovers the typical Bingham-like behavior that is observed in (strong) conventional MR fluids at experimentally accessible times.

Electrokinetic energy conversion in microchannels using polymer solutions.

Electrokinetic energy conversion in microfluidic systems is a subject of intense research at present, where the main objective is to improve the thermodynamic efficiency of the process. As a novel strategy to the problem, this work focuses on the fluid dynamic properties of the working fluid. It is shown that polymer solutions with wall depletion can substantially increase the conversion efficiency in comparison to simple electrolytes under the same operating conditions. The effect is given by a reduction of the hydrodynamic conductance, while the streaming current is unaltered. It is also found that the maximum efficiency of electrokinetic power generation differs from that of electroosmotic pumping, in contrast to the case of simple electrolytes. This is due to the non-Newtonian character of polymeric fluids, which leads to nonlinear electrokinetic relations.

Gel transition of depletion flocculated emulsions

Abstract The gel transition of depletion flocculated oil-in-water emulsions is analyzed as a function of the strength of the droplet–droplet interaction. The critical volume fraction for gelation has been found to decrease exponentially with the magnitude of the interaction energy at contact, in agreement with recent proposals for aggregating colloids. The viscosity modeling of these emulsions had been discussed in a previous work (Colloids Surf. A 203 (2002) 11). The results reported here show the interplay between phase behavior and rheology of flocculated emulsions.

Optofluidic characterization of nanoporous membranes.

An optofluidic method that accurately identifies the internal geometry of nanochannel arrays is presented. It is based on the dynamics of capillary-driven fluid imbibition, which is followed by laser interferometry. Conical nanochannel arrays in anodized alumina are investigated, which present an asymmetry of the filling times measured from different sides of the membrane. It is demonstrated by theory and experiments that the capillary filling asymmetry only depends on the ratio H of the inlet to outlet pore radii and that the ratio of filling times vary closely as H(7/3). Besides, the capillary filling of conical channels exhibits striking results in comparison to the corresponding cylindrical channels. Apart from these novel results in nanoscale fluid dynamics, the whole method discussed here serves as a characterization technique for nanoporous membranes.

The apparent hydrodynamic slip of polymer solutions and its implications in electrokinetics

The apparent hydrodynamic slip of polymer solutions is a result of polymer depletion at channel walls, and its fluid dynamic effects are well known. This work reviews the evidences of apparent slip in electrokinetics, and discusses practical consequences in the following fields: (i) electrokinetic transport of polymer solutions in microchannels, which is of interest for the design and operation of microfluidic chips, particularly for electrophoresis; (ii) electroosmotic pumping, where it has been observed that the employment of polymer solutions greatly enhances the output pressure; and (iii) electrokinetic energy conversion, where the apparent slip also contributes to improve the conversion efficiency. In all cases, critical discussions are taken from basic physical concepts.