(2ga) Active Matter and Liquid Crystals Under External Fields: Basic Science and Applications

Complex fluids with anisotropic building blocks are widespread in nature spanning diverse length scales. Examples are chemically interacting molecules to create DNA macromolecules, self-assembled phospholipids for cell membranes, or living organisms’ tissues with striated muscles. These thread-like morphologies bring structural support with essential mechanical responses that give functionality and adaptability to external changes.

Understanding these basic structures and self-assembly mechanisms serves to develop a new generation of synthetic/hybrid materials. Non-trivial rheological and optical responses are found when viscoelasticity is essential, tuned by concentration, ionic strength, and thermal energy. Even more, liquid crystal phases can be formed, where surface and elastic interactions are crucial, coupled with the formation of topological defects.

The study of thread-like fluids is a significant multidisciplinary area of research that combines the knowledge of physicists, chemists, engineers, biologists, and experts in medical disciplines, with applications in the food, pharmacy, sensor, displays, cosmetics, or medical industries, among others.

On the other hand, there are significant advances in mimicking the mechanisms of actuation of active systems found in nature. Using living organisms such as bacteria or synthetic colloids embedded in complex fluids, it is possible to add energy to these systems using external fields, ATP, or chemotactic reactions, to transform it into mechanical work. The richness of these active materials relies on the system's diverse combinations of active elements for different outcomes.

Combining the knowledge from complex fluids and active matter, various systems with diverse functionalities can be engineered.

Keeping with this theme, my postdoctoral research has been focused on:

• The experimental study of liquid crystalline mesophases in thermotropic materials, particularly activating inclusions driven by the local nematic-isotropic transitions triggered by light. Recent work regards the local activation of quasi-2D platelets that move under light illumination. This system’s Different facets depend on the 2D or 3D platelet's confinement, such as focal conic domains during motion. Other explored systems include spherical colloids (silica and Janus Ti-coated) whose motility is triggered by the mentioned mechanisms but with different outcomes, depending on their concentration in liquid crystal suspensions. Theoretical stochastic models to understand the dynamics are now under exploration and continuum field-theory simulations.

• Active lyotropic liquid crystals. Microtubules (tubulin) and kinesin molecular motors generate extensional stresses fueled by ATP, creating chaotic motion of the material. Continues creation and annihilation of topological defects occur, triggered by the activity and elasticity of the system. Designing interconnected 3D-printed microfluidic channels permits the confinement of the material with directed flow losing its chaotic nature. The goal consists of creating a series of active flow networks to perform logical operations with the fluid.

• Crystallization process on curved surfaces of liquid crystal-blue phase core-shell droplets suspended in water. Blue phases represent a three-dimensional assembly of chiral liquid crystal molecules arranged into body-centered or simple cubic symmetries. Optical and confocal microscopy help us elucidate textures with variable temperature dependence structures. Continuum field-theory simulations predict the most stable structures.

• Characterizing the synergistic effect of adding azobenzene molecules as a building block for assembling worm-like surfactant micelles. We implemented mechanical rheology, Raman spectroscopy, x-ray scattering, cryo-TEM imaging, and molecular dynamic simulations. A comparison is made with well-known worm-like micelles.

My research group will be tackling issues taking three interconnected initial directions:

• Active inclusions within passive and active liquid crystals with directed motion

Active transport of materials in living systems happens at the cellular level, within the plasmatic membrane, in which proteins and other receptors move. The dynamics need to be clearly understood and can be affected by various factors. However, controlled experiments of active liquid crystal colloids within the nematic or smectic phases can help to understand the strength of the interactions occurring when the continuous media is highly structured. Different activation mechanisms can be implemented, including light, electric fields, and chemotaxis reactions. Variations in the colloids' geometry, material, the confinement's size, and the design of obstacles in the liquid crystal sample can lead to different dynamics and optical responses. Forcing a distortion of the liquid crystal director treating the confinement walls with photo-switchable molecules can add an extra level of complexity to control the motion of the inclusions. Another possibility is the confinement of passive colloids within active liquid crystals, with the indirect use of ATP as fuel. The active length scale would determine the colloids’ dynamics and lead to a clever selection of geometries.

• Pursuing collaborations: design of highly susceptible liquid-crystalline sensors.

In recent years, liquid crystals have been used to develop sensors for detecting antigen molecules to diagnose and early detecting illnesses. It has been proved that the geometrical confinement of the liquid crystal mesogens within spherical boundaries improves the detection given the formation of topological defects and elastic distortions. However, it is interesting to engineer new methods of detection using active liquid crystal colloids. The dynamics and organization of the colloids can be affected by the presence of spurious molecules, and diverse external fields can be used to activate and organize the colloids. The concentration of the spurious molecules can be characterized using the optical response and the diffusivity of the colloids. Numerical simulations can help to complete the characterization.

• Thermotropic liquid crystal colloid arrays driven by optical tweezers to control the formation of defects at the nematic-isotropic interface.

It is well known that the presence of colloids generates distortions in their liquid crystal matrix, leading to the appearance of birefringence responses when using crossed polarizers. There are prolific advances in designing the formation of defects and liquid crystal distortions in the presence of collections of colloids with diverse patterns. However, more observations of these effects must be made when the colloids work as local heat sources (local light absorbers) activated by external illumination. With sufficient energy, the local absorption of light generates a nematic-isotropic phase transition resulting in interacting local isotropic bubbles. Preliminary tests show the formation of topological defects with different tunable configurations for different colloidal arrangements. Collective elastic and surface interactions at the nematic-isotropic interface level are crucial. Potential photonic applications are expected.

Active collaborations with groups from the same or other institutions are expected.

Teaching interests

Good education and mentoring are primordial to recognizing the students’ weaknesses and strengths. The scientists must educate as part of their roles to benefit the community, even more than just paying back all the efforts our mentors put into us. It is equally important to educate and mentor students pursuing a STEM career and students in elementary levels of school through education and outreach programs. These programs help to decrease the significant gaps our communities suffer from economic and social points of view. As part of the teaching roles, my interests are elementary physics (mechanics, electrodynamics, optics), statistical mechanics, and liquid crystals. I am also interested in participating in outreach programs and coordinating with former colleagues.

My mentoring, education, and outreach experiences are listed as follows:

UChicago Pritzker School of Molecular Engineering

• Science Clubs (Clubes de Ciencia) instructor. A collaborative effort between Mexican and US institutions.
• Mentoring and lab training of two undergraduate students.
• Postdoc Diversity and Inclusion Certificate Training.
• Postdoc Mentoring Certificate Training.

UNAM Faculty of Sciences

• Teaching assistant, undergraduate level: Deformable media.
• Teaching assistant, undergraduate level: Introduction to Quantum Physics.
• Teaching assistant, undergraduate level: Introduction to Quantum Physics.

UNAM Institute of Physics

• Teaching assistant, graduate level: Statistical Physics.

UMSNH Faculty of Physics and Mathematics Sciences

• Trainer of the participants in the State and XIX National Physics Olympiad.
• Trainer of the participants in the State and XVIII National Physics Olympiad.

Background

I was born in Morelia, Mexico. I obtained my BS in physics and mathematics from the University of Michoacán (UMSNH). After that, I earned an MS in physics in 2014 at the Institute of Physics from the National University of Mexico (UNAM). At the same place, in 2019, I earned a Ph.D. degree in physics, with honors, focused on the experimental study of mechanical and dynamical responses of thread-like morphologies in complex fluids by rheology, microrheology with dynamic light multi-scattering, and static scattering techniques. Different systems were studied, including carbon nanotubes-poly electrolyte pH tunable gels, worm-like micelles made of amphiphilic block-copolymers with tunable rheological responses by varying the size of the hydrophilic chain, and worm-like surfactant micelles with tunable rheological responses when adding a photo-switchable molecule. I joined Prof. Juan de Pablo’s group at the University of Chicago in March 2019. From February to October 2023, I conducted a research stay at the ESPCI-PSL Paris under the supervision of Prof. Teresa Lopez-Leon. At the ESPCI, I worked with active nematic systems confinement within 3D-printed microfluidic channels.