Papers by Teresa Lopez-Leon
Physical Review E, Feb 26, 2009
Liquid shells (e.g., double emulsions, vesicles, etc.) are susceptible to interfacial instability... more Liquid shells (e.g., double emulsions, vesicles, etc.) are susceptible to interfacial instability and rupturing when driven out of mechanical equilibrium. This poses a significant challenge for the design of liquidshell-based micromachines, where the goal is to maintain stability and dynamical control in combination with motility. Here, we present our solution to this problem with controllable self-propelling liquid shells, which we have stabilized using the soft topological constraints imposed by a nematogen oil. We demonstrate, through experiments and simulations, that anisotropic elasticity can counterbalance the destabilizing effect of viscous drag induced by shell motility and inhibit rupturing. We analyze their propulsion dynamics and identify a peculiar meandering behavior driven by a combination of topological and chemical spontaneously broken symmetries. Based on our understanding of these symmetry breaking mechanisms, we provide routes to control shell motion via topology, chemical signaling, and hydrodynamic interactions.
Journal of Physical Chemistry C, Sep 20, 2008
The original Hofmeister series concerned the effectiveness of different salts in precipitating ne... more The original Hofmeister series concerned the effectiveness of different salts in precipitating negatively charged proteins. Although the relative position of the species in the series has been demonstrated to be almost universal, reversed Hofmeister sequences have been observed in many systems. Most of them are directly related with the sign of charge (positive or negative) of the surface studied. In this paper, we show experimental evidence of alterations in the Hofmeister order induced not only by changing the sign of the surface charge, but also by varying the hydrophobic/hydrophilic character of the surface. We have worked with a wide variety of systems showing different hydrophilic/hydrophobic character and having a pH-dependent charge sign. Particularly, the effect of these two factors (the surface charge and the hydrophilicity/phobicity) on the ion ranking has been analyzed by studying the colloidal stability of the systems. The typical inversions induced by modifying the charge sign of the particle were observed; in addition and more noteworthy, the direct series observed with hydrophobic systems turned into reversed series as the surface became more hydrophilic for surfaces of identical sign of charge. The correlation found between these inversions and restabilization processes promoted by hydration forces at high salt concentrations evidenced the solvent-structural nature of such inversions. Useful conclusions about the origin of these specific ion effects have been drawn from a comparative analysis among our systems.
Langmuir, Dec 8, 2004
The hydration interaction is responsible for the colloidal stability observed in protein-coated p... more The hydration interaction is responsible for the colloidal stability observed in protein-coated particles at high ionic strengths. The origin of this non-DLVO interaction is related not only to the local structure of the water molecules located at the surface but also to the structure of those molecules involved in the hydration of the ions that surround the colloidal particles. Ruckenstein and co-workers have recently developed a new theory based on the coupling of double-layer and hydration interactions. Its validity was contrasted by their fitting of experimental data obtained with IgG-latex particles restabilized at high salt concentration. The theory details the important role played by the counterions in the stability at high salt concentrations by proposing an ion pair reaction forming surface dipoles. These surface dipoles are responsible of repulsive interactions between two approaching surfaces. This paper checks the theory with recent data where some ions associated with the Hofmeister series (NO3-, SCNand Ca 2+) restabilize the same kind of IgG-latex systems by means of hydration forces. Surprisingly, these ions induce stability acting even as co-ions, likely by modifying the water structure at the surface, but not forming surface ion pairs. Therefore, this experimental evidence would question Ruckenstein's theory based on the surface dipole formation for explaining the observed restabilization phenomena.
Soft Matter, 2022
In this work, we investigate the possibility of inducing valence transitions, i.e. transitions be... more In this work, we investigate the possibility of inducing valence transitions, i.e. transitions between different defect configurations, by transforming a nematic shell into a nematic droplet. Our shells are liquid crystal droplets containing a smaller aqueous droplet inside, which are suspended in an aqueous phase. When osmotically de-swelling the inner droplet, the shell progressively increases its thickness until it eventually becomes a single droplet. During the process, the shell energy landscape evolves, triggering a response in the system. We observe two different scenarios. Either the inner droplet progressively shrinks and disappears, inducing a defect reorganization, or it is expelled from the shell at a critical radius of the inner droplet, abruptly changing the geometry of the system. We use numerical simulations and modeling to investigate the origin of these behaviors. We find that the selected route depends on the defect structure and the energetics of the system as it evolves. The critical inner radius and time for expulsion depend on the osmotic pressure of the outer phase, suggesting that the flow through the shell plays a role in the process.
Physical Review X, Nov 1, 2017
Chirality, ubiquitous in complex biological systems, can be controlled and quantified in syntheti... more Chirality, ubiquitous in complex biological systems, can be controlled and quantified in synthetic materials such as cholesteric liquid crystal (CLC) systems. In this work, we study spherical shells of CLC under weak anchoring conditions. We induce anchoring transitions at the inner and outer boundaries using two independent methods: by changing the surfactant concentration or by raising the temperature close to the clearing point. The shell confinement leads to new states and associated surface structures: a state where large stripes on the shell can be filled with smaller, perpendicular sub-stripes, and a focal conic domain (FCD) state, where thin stripes wrap into at least two, topologically required, double spirals. Focusing on the latter state, we use a Landau-de Gennes model of the CLC to simulate its detailed configurations as a function of anchoring strength. By abruptly changing the topological constraints on the shell, we are able to study the interconversion between director defects and pitch defects, a phenomenon usually restricted by the complexity of the cholesteric phase. This work extends the knowledge of cholesteric patterns, structures that not only have potential for use as intricate, self-assembly blueprints but are pervasive in biological systems.
Journal of Physical Chemistry B, Feb 3, 2006
A cationic and an anionic poly(N-isopropylacrylamide) (poly(NIPAM)) microgel latex were synthesiz... more A cationic and an anionic poly(N-isopropylacrylamide) (poly(NIPAM)) microgel latex were synthesized via batch radical polymerization under emulsifier-free conditions. The hydrodynamic properties, colloidal stability, and electrokinetic characteristics of these two samples were studied. The hydrodynamic particle size variation was discussed by considering the effect of salinity and temperature on the shrinkage of the thermally sensitive polymer domains. The colloidal stability also depended on temperature and electrolyte concentration. A stability diagram with two well-defined domains (stable and unstable) was obtained. The flow from one domain to the other was fully reversible due to the peculiar (de)hydration properties of the polymer. The electrokinetic behavior, which depends on electrical and frictional properties of the particles, was analyzed via electrophoretic mobility measurements. Results were discussed by considering both the particle structure dependence on temperature and salinity, and the electric double layer compression. In addition, the electrophoretic mobility data were analyzed using Ohshima's equations for particles covered by an ion-penetrable surface charged layer, as well as using another simpler equation for charges located on a hydrodynamic equivalent hard sphere. Differences between the properties of both latexes were justified by the presence of a hydrophilic comonomer, aminoethyl methacrylate hydrochloride (AEMH), in the cationic microgel.
Bulletin of the American Physical Society, Mar 20, 2009
Submitted for the MAR09 Meeting of The American Physical Society Hybrid shells of nematic liquid ... more Submitted for the MAR09 Meeting of The American Physical Society Hybrid shells of nematic liquid crystal ALBERTO FERNANDEZ-NIEVES, TERESA LOPEZ-LEON, Georgia Tech-We investigate the consequences of changing the boundary conditions for the nematic director at the outer surface of a spherical shell from planar to homeotropic. We find there are different routes to the final equilibrium configuration, depending on the initial shell structure. For bipolar shells, which are shells having two pairs of s=+1 boojums on either surface, a disclination ring forms, shrinks and disappears in a process that is highly reminiscent of that seen in bipolar drops. By contrast, shells with four s=+1/2 defects develop open disclination lines in the inner surface; these lines form between the original s=+1/2 defects and force their approach and coalescence. These results highlight the fascinating range of behaviors that are driven by the interplay between topological constraints and the nematic order of liquid crystals.
Soft Matter, 2020
The role of applied fields on the structure of liquid crystals confined to shell geometries has b... more The role of applied fields on the structure of liquid crystals confined to shell geometries has been studied in past theoretical work, providing strategies to produce liquid crystal shells with controlled defect structure or valence. However, the predictions of such studies have not been experimentally explored yet. In this work, we study the structural transformations undergone by tetravalent nematic liquid crystal shells under a strong uniform magnetic field, using both experiments and simulations. We consider two different cases in terms of shell geometry and initial defect symmetry: i) homogeneous shells with four s = +1/2 defects in a tetrahedral arrangement, and ii) inhomogeneous shells with four s = +1/2 defects localized in their thinner parts. Consistently with previous theoretical results, we observe that the initial defect structure evolves into a bipolar one, in a process where the defects migrate towards the poles. Interestingly, we find that the defect trajectories and dynamics are controlled by curvature walls that connect the defects by pairs. Based on the angle between Bs, the local projection of the magnetic field on the shell surface, and n + 1 2 , a vector describing the defect orientations, we are able to predict the nature and shape of those inversion walls, and therefore, the trajectory and dynamics of the defects. This rule, based on symmetry arguments, is consistent with both experiments and simulations and applies for shells that are either homogeneous or inhomogeneous in thickness. By modifying the angle between Bs and n + 1 2 , we are able to induce, in controlled way, complex routes towards the final bipolar state. In the case of inhomogeneous shells, the specific symmetry of the shell allowed us to observe a hybrid splay-bend Helfrich wall for the first time.
Hofmeister Effects on Poly(NIPAM) Microgel Particles: Macroscopic Evidence of Ion Adsorption and Changes in Water Structure
ChemPhysChem, Jan 8, 2007
The term Hofmeister effects is broadly used to refer to ionic specificities in many different phy... more The term Hofmeister effects is broadly used to refer to ionic specificities in many different physical, chemical and biological phenomena. The origin of this ionic specificity is sought in two interdependent microscopic sources: 1) the peculiarities of the solvent structure near surfaces and around the ions, and 2) specific ion adsorption-exclusion mechanisms near a surface. In this work, Hofmeister effects on poly(N-isopropylacrylamide) [poly(NIPAM)]-based microgels are examined. Poly(NIPAM) particles are thermally sensitive microgels exhibiting volume-phase transitions with temperature. This temperature-sensitive system seems to be suitable for the independent observation of the two microscopic sources of Hofmeister effects. On the one hand, volume-phase transition, evaluated by photon correlation spectroscopy (PCS), gives information about how the presence of ions changes the water structure around the poly(NIPAM) chains. On the other hand, electrokinetic studies show relevant data about ionic adsorption-exclusion phenomena at the polymer surface.
Active nematics near walls
Bulletin of the American Physical Society, Mar 15, 2021
Frontiers in Physics, Sep 11, 2020
Defect dynamics on active nematic ellipsoids
Bulletin of the American Physical Society, Mar 15, 2021
Interactions between proteins and cellulose in a liquid crystalline media: Design of a droplet based experimental platform
International Journal of Biological Macromolecules
Spontaneous Self-Constraint in Active Nematic Flows
arXiv (Cornell University), Jun 8, 2023
Bulletin of the American Physical Society, Mar 5, 2015
Submitted for the MAR15 Meeting of The American Physical Society Toplogical defects in cholesteri... more Submitted for the MAR15 Meeting of The American Physical Society Toplogical defects in cholesteric liquid crystal shells ALEXANDRE DARMON, EC2M
Salt Effects in the Cononsolvency of Poly(N-isopropylacrylamide) Microgels
ChemPhysChem, Jan 18, 2010
Poly(N‐isopropylacrylamide) (PNIPAM) is well known to exhibit reentrant behavior or cononsolvency... more Poly(N‐isopropylacrylamide) (PNIPAM) is well known to exhibit reentrant behavior or cononsolvency in response to the composition of a mixed solvent consisting of water and a low‐chain alcohol. Since the solvent structure plays an important role in this phenomenon, the presence of structure‐breaking/structure‐making ions in solution is expected to have a dramatic effect on the cononsolvency of PNIPAM. The present work examines the way that the presence of different salts can modify the reentrant‐phase diagram displayed by a cationic PNIPAM microgel in the mixed ethanol/water solvent. The effects of four Hofmeister anions—SO42−, Cl−, NO3− and SCN−—with different abilities to modify the solvent structure are analyzed. The species with kosmotropic or structure‐making character show a clear competition with ethanol for the water molecules, intensifying the nonsolvency of the PNIPAM with the EtOH volume fraction (ϕe). However, striking results are found with the most chaotropic or structure‐breaking anion, SCN−. In contrast to what happens in water‐rich solutions, the presence of SCN− in alcohol‐rich solvents enhances the solubility of the polymer, which macroscopically results in the microgel swelling. Moreover, this ion displays great stabilizing properties when ϕe&gt; is 0.2. These results have been explained by considering how chaotropic or structure‐breaking ions interact with water and ethanol molecules.
Small, Oct 10, 2019
We present a structurally reversible smectic liquid crystal (LC) emulsion made of semifluorinated... more We present a structurally reversible smectic liquid crystal (LC) emulsion made of semifluorinated rod-type molecules in silicon oil, which is controlled by simple heating and cooling. Without adding any kind of additives, such as surfactants, polymers or emulsifiers, and without using any special tools, such as microfluidics or gas bubbling, the LC molecules spontaneously form monodisperse spherical and myelin-like structures upon cooling from the isotropic temperature. The LC emulsion can easily trap guest materials, providing a platform for repeatable and reliable switchable emulsification. For example, this interesting system enables to realize an on-off lasing system by confining fluorescent dyes in the LC droplets.
Physical review, Dec 5, 2016
We present a novel, self-consistent and robust theoretical model to investigate elastic interacti... more We present a novel, self-consistent and robust theoretical model to investigate elastic interactions between topological defects in liquid crystal shells. Accounting for the non-concentric nature of the shell in a simple manner, we are able to successfully and accurately explain and predict the positions of the defects, most relevant in the context of colloidal self-assembly. We calibrate and test our model on existing experimental data, and extend it to all newly observed defects configurations in chiral nematic shells. We perform new experiments to check further and confirm the validity of the present model. Moreover, we are able to obtain quantitative estimates of the energies of +1 or +3/2 disclination lines in cholesterics, whose intricate nature was only reported recently.
Defect coalescence in spherical nematic shells
Physical Review E, Sep 28, 2012
ChemPhysChem, May 3, 2012
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Papers by Teresa Lopez-Leon