My research has been devoted to the investigation of equilibrium and non-equilibrium properties of Soft Matter systems:
- Study of non-equilibrium processes, such as colloidal aggregation in 2 and 3 dimensions, simultaneous sedimentation/aggregation, colloidal deposition processes, including the hydrodynamic effects involved in these processes.
- Formation of nanostructures in charged colloidal systems
- Study of effective interactions (such as depletion forces), structure, phase behavior and interfacial properties of mixtures of nanoparticles.
- Study of structure, interactions involved in charged thermoresponsive microgel particles.
- Study of the permeation of ions and charged/neutral cosolutes inside microgel particles.
Effective interactions in microgel suspensions (current research)
Understanding the equilibrium and the non-equilibrium structure, dynamics and local distributions of the different components of a colloidal suspension in terms of the inter-particle interactions is a challenge of great importance in Soft Matter or Colloidal Physics. In this topic, we focus on colloidal systems formed by an aqueous suspension of neutral and charged microgels.
A microgel (or nanogel) particle is formed by a cross-linked polymer network of colloidal size immersed in a solvent, which can be designed to swell or shrink in response to many external parameters, such as temperature, pH, and solvent quality among others. Due to their nanometric size, the timescale of the swelling response (which is roughly proportional to the square of the typical spatial dimension of the microgel) is of the order of seconds, which is very short compared to the ones observed in the so-called macroscopic gels. Furthermore, the soft and porous nature of the microgels allow them to be permeated by the solvent, ions and other neutral or charged macromolecules. The combination of these properties make microgel suspensions unique smart materials for industrial and biomedical applications, such as carrier particles for biomolecules or controlled drug release, filtration or stimuli controlled nanoreactors
We are currently investigating the stability and structure of microgel dispersions, the degree of swelling, and the permeation of different kind of solutes (small proteins, reactants, ions, drugs and genetic material) into the microgel, and relate all these properties to the effective pair microgel-microgel and microgel-solute interactions. In particular, we use theoretical methods and coarse-grained computer simulations to investigate the role of the electrostatic, excluded-volume, water-mediated hydrophobic/hydrophilic, dispersive and elastic interactions. Concerning theory, we make use of methods based on Ornstein-Zernike integral equations, density functional theories for complex fluids and other thermodynamic and statistical-mechanical approaches. The combination of these procedures allows us to predict the degree of solute sorption onto the microgel and to determine the effective interactions between the microgel particle and the solute. On the other hand, microgels behave as model particles to explore collective effects (structure formation, dynamics and rheology) in dilute and dense dispensions of particles in the bulk or confined at fluid-fluid interfaces, and to investigate the effective microgel-microgel interactions involved in both geometries.
These investigations and the future work is being financially supported by the following research projects founded by the Spanish Ministerio de Economía y Competitividad (MINECO), “Plan Nacional de Investigación, Desarrollo e Innovación Tecnológica (I+D+i)”
Project FIS2016-80087-C2-1-P: “Interactions and collective properties of nanogel/microgel-based soft-matter systems of nanotechnological interest”, 2017-2019
Project MAT2012-36270-C04-02: “Structure and interactions in systems of soft nanoparticles (nanogels and liposomes)”, 2013-2015
Hydrodynamics in sedimenting suspensions
Many industrial applications of colloidal suspensions depend critically on their behavior under non-equilibrium conditions. Such properties are, however, notoriously hard to calculate because of the long-ranged solvent induced hydrodynamic interaction. Partially for this reason, the vast majority of theoretical and computational treatments of the nonequilibrium regime have focused on hard-sphere particles. Thus our understanding of how attractive interparticle interactions affect the nonequilibrium behavior of colloidal suspensions is still in its infancy. This state of affairs stands in marked contrast to the equilibrium regime, where methods to calculate how interactions control phase behavior and interfacial properties are already well developed.
To address this fundamental question, we study the steady-state sedimentation of spherical particles through a viscous solvent at low Reynolds number using Stochastic Rotation Dynamics computer simulations. Besides its intrinsic interest for statistical mechanics, sedimentation is also important for understanding industrial applications such as paints, coatings, ceramics, food, and cosmetics. We also investigate the role that interparticle attractive interactions (reversible aggregation) have on the sedimentation rate and on the hydrodynamic fluctuations,, and how increasing the particle concentration enhances two competitive effects, namely cluster formation (which accelerates the sediemntation rate) and the hydrodynamic backfklow retardation.
Depletion interactions in binary colloidal systems
The interaction between two nanoparticles in solution can be qualitatively modified by the presence of smaller suspended particles. For example, the addition of nonadsorbing polymer in a good solvent can lead to attractive depletion interactions, as first explained over 50 years ago by Asakura and Oosawa. This depletion effect arise when two nanoparticles approach each other so that their exclusion volumes overlap, resulting in more free volume available for the polymer chains. The subsequent encrease of entropy translates into an effective depletion attraction between the two nanoparticles. The strength of the depletion attraction is roughly controlled by the polymer concentration, and the range of the attractive well is mainly determined by the size of the polimers. The depletion can cause important effects on the statics and dynamics of phase transitions, including fluid-fluid demixing, crystallization, gelation, or glass transitions, as well as affect the interfacial properties associated with phase coexistence.
In addition to depletion forces, another sort of effective interactions can emerge depending on whether the small component is attracted or repelled to the surface of the big nanoparticle. For the case of strong attraction, the small component tends to be attached to the surface. Therefore, when two large nanoparticles approach each other, the adsorbed layers of small ones overlap yielding effective repulsive forces between the large nanoparticles that stabilize the suspension. In the opposite case, i. e. long-range repulsion between big and small components, the small particles are excluded in the proximity of the big particle surfaces. Consequently, within the approach of a pair of big colloids, the overlap of the depletion regions induces a difference in osmotic pressure that is translated into an enhanced effective attraction. In this balance, the interaction between small particles plays also an important role, leading to other mechanisms such as bridging or repulsion though attraction effects.
Aggregation and heteroaggregation processes
Colloidal systems are composed of particles which are large compared to the solvent molecules, but still small to exhibit thermal motion. Under inestable conditions (particle interactions dominated by attractions) the particles stick on contact due to attractive forces forming larger clusters. Depending of the strength and range of such attracions, the aggregation may be clasified in different non-equilibrium regimes: diffusive-limited cluster aggregatio, reaction limited cluster aggregation, reversible aggregation and fragmentation,…
Aggregation of colloidal particles occurs in a wide variety of physical, chemical and biological processes. Hence, it is of great practical interest to predict the time evolution of the aggregating species from relatively simple theoretical expressions. Smoluchowski’s equation has been widely used for this purpose. This equation, however, needs a physically deduced aggregation kernel before meaningful conclusions may be drawn from its predictions. For this purpose, this equation is complemented with diffusion theory combined with concepts of fractal geometry and computer simulations, which have been the most important tools for describing kinetic and structural aspects of aggregation processes.