Research

Interaction of colloids with laser-induced external fields (in collaboration with Jose Luis Arauz, Stefan Egelhaaf, Ramon Castañeda and Ivan Guerrero), students: Juan Manuel Molina, Norma Palmero

Any inhomogeneous distribution of light can act as an external field for a colloidal suspension. We are exploring using such laser-induced external fields not only for understanding Brownian motion, but also as a tool for giving a new insight in a variety of phenomena, such as diffusion in non-hydrodinamic confinement, fluctuations in systems formed by a small number of particles, first-time-passage problems, diffusion in non-equilibrium conditions (time and spatially dependent external potentials) and hydrodynamic correlations in solvent near the critical point.

Recently, in collaboration with Stefan Egelhaaf lab, we are now studying the dynamics of planar colloidal molecules in periodic potentials, using not only the ray tracing algorithm but also random walker simulations where the effect of confinement is simulated using a markovian process of the escape of the particle inside a cage.

Active matter, students: Natalia Rincon

This is a very important topic nowadays in soft matter and thus the main project of Natalia is to explore the dynamic scenario of active particles (either spherical or anisotropic) trapped optical tweezers or in general in inhomogeneous distribution of light. We are synthetizing Janus active particles as shown in the seminal work of Bechinger and Ivo (i,e, partially covered carbon particles in a solvent close to the critical transition) and analyzing the de-mixing geometry depending on the boundary conditions first.

Colloidal dynamics of anisotropic colloids under confinement (in collaboration with Jose Luis Arauz, Ramon Castañeda and Stefan Egelhaaf)

Understanding the dynamics of complex Brownian particles is fundamental for several practical problems, such as biodistribution, sedimentation, coagulation, flotation and rheology. From a more fundamental point of view, dealing theoretically with anisotropic colloids is intrinsically complicated due to the additional degrees of freedom required to describe this phenomena. Experimentally, synthesis of large amounts of monodisperse anisotropic colloids remains a challenge. We are currently studying the Brownian motion of the simplest anisotropic colloid, i.e. two spherical particles fused together (a colloidal dumbbell). Previously, the static properties of mixtures of spherical and dumbbells particles was studied, along with a well establish method for synthesis of this anisotropic colloid . Nowadays, we are exploring the effects of confinement, both in static and dynamic properties, in a colloidal mixture of spherical particles and dumbbells, ranging from the dilute regime to a concentration close to the crystallization. In collaboration with Ramon Castañeda’s group, from University of Guanajuato, molecular dynamics simulations are being used to test our experimental results. Additional experimental evidence is also being obtained along with Prof. Egelhaaf group.

Optical tweezers for biomedical applications (in collaboration with Beatriz Morales and Vanesa Olivares)

Quite recently we develop an experimental set-up capable of trapping several particles at the same time using holographic optical tweezers. With this tool, two particles attached to a cell can be trapped and moved, thus applying a deformation on the cell. Using the proper calibration, the restoring forces, commonly in the order of pico newtons, can be measured. In the context on a recently accepted CONACYT funding, we are now linking the mechanical properties of the cell with the cytoskeleton dynamics using a combination of optical tweezers and fluorescence microscopy.

We also have a close collaboration with Prof. Carmenza Spadafora's lab for studying the mechanical properties of Red Blood Cells infected with Malaria

Microrheology of complex fluids (in collaboration with Jose Luis Arauz and Rolando Castillo)

The Brownian motion of a particle embedded in a complex fluid is quite different from the case of a newtonian fluids. In particular, different time regimes appear in the temporal evolution of the Brownian motion, finding, for example, sub-diffusive regimes in the mean squared displacement. This difference is attributed to the interaction of the particle with the complex fluid. During my PhD thesis I used a dynamic light scattering technique to measure the dynamic properties of colloidal particles embedded in a variety of complex fluids (wormlike micelles, polymeric solutions, virus suspensions) and compare their microrheological properties.

Nowadays we am also exploring the microrheological measurements in polymeric suspensions just above the crossover concentration, where depletion effects are reported to be important, and also comparing our results with the zeroth order theory of colloid-polymer interaction, the asakura-oosawa model. We are also analyzing the effects of viscoelasticity on microenvironments.

Light Propagation in turbid media (in collaboration with Beatriz Morales)

A homogeneous media is transparent due to rectilinear propagation of rays, or, under the vision of electromagnetic waves, due to wave front propagation without deformation. A colloidal suspension, in the opposite, is turbid, meaning that light is not propagating in a rectilinear way anymore. This effect is called scattering, and is commonly found in inhomogeneous materials, i.e. in materials with position-dependent refractive index. In a turbid media, direction of propagation of light continuously changes, and in some cases, molecules absorb part of the incident energy. In dynamic light scattering techniques, light exiting a colloidal suspension is analyzed and dynamical properties of the scattering particles (colloidal particles) are extracted from the time evolution of the scattered intensity. However, this kind of techniques requires a model for light propagation, and also requires mesoscopic properties that characterize the scattering process, such as the so called optical properties. We are currently using models for light propagation, such as GPU accelerated Monte Carlo simulations, to develop techniques that could allow the extraction of optical properties required in dynamic light scattering techniques, and also exploring the possibilities of more general light scattering techniques, not only in complex fluids applications, but also in biomedical applications.