arXiv subjects

Francesca Serra

Publications and source records attributed to Francesca Serra.

10 recordsLinked to original sources

arXiv2025

Surface functionalization modulates collective cell behavior at integer topological defects

Living cells establish long-range orientational order through collective alignment, giving rise to topological defects whose functional relevance is increasingly recognized in tissue organization and morphogenesis. Engineered topographical patterns have been used to induce such defects in cell monolayers, mimicking natural biological phenomena. In this work, we investigate the effect of cell-surface adhesion on collective cell dynamics at a vortex integer topological defect imposed by a topographical ring pattern. Adhesion strength is controlled via surface functionalization with poly-D-lysine, fibronectin, or covalently bonded fibronectin, and quantified using atomic force microscopy. As surface chemistry is modified, cell morphology changes from irregular to spindle-like, and two distinct collective modes emerge: weakly adhered cells exhibit strong inward motion, while strongly attached cells move tangentially to the ring. Spindle-shaped cells exhibit higher nematic order and promote the emergence of two +1/2 topological defects in the monolayer. We further characterize collective cell dynamics by analyzing correlation lengths and demonstrate the scaling of number density fluctuations in cell systems.

cond-mat.soft↗

arXiv2025

Cells around the corner

The study of spindle-like cells as nematic liquid crystals has led to remarkable insights in the understanding of tissue organization and morphogenesis. In the characterization of this anomalous liquid crystal material, we focus on the energetic cost of splay and bend deformations, in order to determine the elastic anisotropy of the material, i.e. the ratio of the elastic constants associated with splay and bend. We explore the behavior of monolayers of cells in proximity to corners, where cells arrange in splay or bend configuration, strongly dependent on the amplitude of the wedge angle. The angle at which splay and bend deformations are equally likely is determined by the ratio between splay and bend elastic constants. We also show that the splay and bend deformations under confinement can be well approximated using equilibrium liquid crystal theory and statistical mechanics. Finally, our data suggest that for fibroblast cells the common approximation of equal bend and splay constant is valid.

cond-mat.soft↗

arXiv2024

Actuation of Cell Sheets in 3D

The alignment of fibers and cells in living tissues affect their mechanical properties and functionality. In this context, one can draw an analogy between tissues and nematic liquid crystal elastomers. We explore this analogy by growing fibroblasts on 2D-patterned substrates and observing the contraction of cell sheets upon detachment from the substrates. When fibroblast sheets detach, they undergo an anisotropic contraction, with maximum contraction along the nematic director, like nematic elastomers do during phase transition. We quantify this anisotropy using substrates patterned with stripes to induce alignment, finding that cell sheets resemble nematic elastomers with negative Poisson ratio. Then, we apply design principles used for programming curvature in nematic elastomers to actuate 3D structures in the detached fibroblast layers, demonstrating an application of these principles and we support the results with simulations. This proof of concept shows the ability to control the 3D shape through 2D patterning in cell layers, leading to promising avenues to program tissues.

cond-mat.soft↗

arXiv2023

Tunable Three-Dimensional Architecture of Nematic Disclination Lines

Disclinations lines play a key role in many physical processes, from the fracture of materials to the formation of the early universe. Achieving versatile control over disclinations is key to developing novel electro-optical devices, programmable origami, directed colloidal assembly, and controlling active matter. Here, we introduce a theoretical framework to tailor three-dimensional disclination architecture in nematic liquid crystals experimentally. We produce quantitative predictions for the connectivity and shape of disclination lines found in nematics confined between two thinly spaced glass substrates with strong planar anchoring. By drawing an analogy between nematic liquid crystals and magnetostatics, we find that: i) disclination lines connect defects with the same topological charge on opposite surfaces, and ii) disclination lines are attracted to regions of the highest twist. Using polarized light to pattern the in-plane alignment of liquid crystal molecules, we test these predictions experimentally and identify critical parameters that tune the disclination lines' curvature. We verify our predictions with computer simulations and find non-dimensional parameters enabling us to match experiments and simulations at different length scales. Our work provides a powerful method to understand and practically control defect lines in nematic liquid crystals.

cond-mat.soft↗

arXiv2022

Nematic colloidal micro-robots as physically intelligent systems

Physically intelligent micro-robotic systems exploit information embedded in micro-robots, their colloidal cargo, and their milieu to interact, assemble and form functional structures. Nonlinear anisotropic fluids like nematic liquid crystals (NLCs) provide untapped opportunities to embed interactions via their topological defects, complex elastic responses, and their ability to dramatically restructure in dynamic settings. Here we design and fabricate a 4-armed ferromagnetic micro-robot to embed and dynamically reconfigure information in the nematic director field, generating a suite of physical interactions for cargo manipulation. The micro-robot shape and surface chemistry are designed to generate a nemato-elastic energy landscape in the domain that defines multiple modes of emergent, bottom-up interactions with passive colloids. Micro-robot rotation expands the ability to sculpt interactions; the energy landscape around a rotating micro-robot is dynamically reconfigured by complex far-from-equilibrium dynamics of the micro-robot's companion topological defect. These defect dynamics allow transient information to be programmed into the domain and exploited for top-down cargo manipulation. We demonstrate robust micro-robotic manipulation strategies that exploit these diverse modes of nemato-elastic interaction to achieve cargo docking, transport, release, and assembly of complex reconfigurable structures at multi-stable sites. Such structures are of great interest to future developments of LC-based advanced optical device and micro-manufacturing in anisotropic environments.

cond-mat.soft↗

arXiv2019

Topological defects of integer charge in cell monolayers

Many cell types spontaneously order like nematic liquid crystals, and, as such, they form topological defects. While defects with topological charge $\pm$1/2 are common in cell monolayers, the defects with charge $\pm$1, relevant in the formation of protrusions and for cell migration in living systems, are more elusive. We use topographical patterns to impose topological charge of $\pm$1 in controlled locations in cell monolayers. We compare the behavior of 3T6 fibroblasts and EpH-4 epithelial cells on such patterns, characterizing the degree of alignment, the cell density near the defects, and their behavior at the defect core. The patterned substrates provide the right conditions to observe isotropic packing of 3T6 cells in the +1 defects. We also observe density variation in the 3T6 monolayers near both types of defects over the same length-scale. These results indicate a defect core size of 150$μ$m-200$μ$m, with isotropic packing possible in the +1 defects and, in other cases, a defect core with rich internal structure.

cond-mat.soft↗

arXiv2018

Tunable colloid trajectories in nematic liquid crystals near wavy walls

The ability to dictate colloid motion is an important challenge in fields ranging from materials science to living systems. Here, by embedding energy landscapes in confined nematic liquid crystals, we design a versatile platform to define colloidal migration. This is achieved by placing a wavy wall, with alternating hills and wells, in nematic liquid crystals, to impose a smooth elastic energy field with alternating splay and bend distortions. This domain generates (meta) stable loci that act as attractors and unstable loci that repel colloids over distances large compared to the colloid radius. Energy gradients in the vicinity of these loci are exploited to dictate colloid trajectories. We demonstrate several aspects of this control, by studying transitions between defect configurations, propelling particles along well defined paths and exploiting multistable systems to send particles to particular sites within the domain. Such tailored landscapes have promise in reconfigurable systems and in microrobotics applications

cond-mat.soft↗

arXiv2016

Experimental realization of the "lock-and-key" mechanism in liquid crystals

The ability to control the movement and assembly of particles in liquid crystals is not only an important route to design functional materials, but also sheds light on the mechanisms of colloidal interactions. In this study we place micron-sized particles with "Saturn ring" defects near a wall with hills and dales that impose perpendicular (homeotropic) molecular anchoring. The strong splay distortion at the wall interacts with the distortion around the particles in the near field and favors their migration towards the dales via the so-called "lock-and-key" mechanism. We demonstrate experimentally that the lock-and-key mechanism can robustly localize a particle at specific topographical features. We observe the complex trajectories traced by the particles as they dock on the dales, estimate the binding energy, and explore a range of parameters that favor or disfavor the docking event, thus exploiting the capabilities of our experimental system. We extend the study to colloids with homeotropic anchoring but with an associated point defect instead of a Saturn ring and show that they find a different preferred location, i.e. we can place otherwise identical particles at well defined sites according to their topological defect structure. Finally, for deep enough wells, confinement drives topological transitions of Saturn rings to dipoles. This ability to tailor wall geometry to guide colloids to well defined sites within nematic liquid crystals represents an important new tool in directed assembly.

cond-mat.soft↗

arXiv2015

Curvature-driven, One-step Assembly of Reconfigurable Smectic Liquid Crystal "Compound Eye" Lenses

Confined smectic A liquid crystals (SmA LCs) form topological defects called focal conic domains (FCDs) that focus light as gradient-index lenses. Here, we exploit surface curvature to self-assemble FCDs in a single step into a hierarchical structure (coined "flower pattern") molded by the fluid interface that is pinned at the top of a micropillar. The structure resembles the compound eyes of some invertebrates, which consist of hundreds of microlenses on a curved interface, able to focus and construct images in three dimensions. Here we demonstrate that these flowers are indeed "compound eyes" with important features which have not been demonstrated previously in the literature. The eccentric FCDs gradually change in size with radial distance from the edge of the micropillar, resulting in a variable microlens focal length that ranges from a few microns to a few tens of microns within a single "flower". We show that the microlenses can construct a composite 3D image from different depth of field. Moreover, the smectic "compound eye" can be reconfigured by heating and cooling at the LC phase transition temperature; its field of view can be manipulated by tuning the curvature of the LC interface, and the lenses are sensitive to light polarization.

cond-mat.soft↗

arXiv2008

Non-linear dynamics of absorption and photo-bleaching of dyes

The celebrated Lambert-Beer law of light absorption in photochromic media is only valid at low intensities of incident light and low concentration of chromophore. Here we address the generic problem of photo-absorption dynamics, experimentally studying the case of azobenzene isomerization. We show that the non-linear regime is very common and easy to achieve in many practical situations, especially in thick samples where the light depletes the chromophore in the first layers and can propagate through the medium with a sub-exponential law. Importantly, the crossover into the non-linear absorption regime only weakly depends on the dye concentration and solution viscosity. We experimentally quantify the characteristics of this peculiar optical response and determine the key transition rate constants.

cond-mat.other↗