arXiv subjects

Saranshu Singla

Publications and source records attributed to Saranshu Singla.

5 recordsLinked to original sources

arXiv2023

Humidity-induced glass transition of a polyelectrolyte brush creates switchable friction in air

Polymer brushes have found extensive applications as responsive surfaces, particularly in achieving tunable friction in solvent environments. Despite recent interest in extending this technology to air environments, little is known about the impact of vapor absorption on friction. Considering polyelectrolyte brushes, we report findings that reveal, with increasing relative humidity, a trend of frictional shear forces decaying over two orders of magnitude only after achieving a critical humidity. However, water absorption, structure, and swelling of the brushes followed continuous trends with increasing humidity. In contrast, a humidity-induced glass transition occurred which caused a shift from dry to fluid-like sliding with the water-swollen brush acting as the lubricant. Below the glass transition, friction is large without regard to water concentration; thus, friction transitions sharply. This switching mechanism, shown to be a general property of the hygroscopic polymer, provides new opportunities for switchable friction or other surface actuation from vapor stimuli.

cond-mat.soft↗

arXiv2022

Mechanism of Structural Colors in Binary Mixtures of Nanoparticle-based Supraballs

Inspired by structural colors in avian species, various synthetic strategies have been developed to produce non-iridescent, saturated colors using nanoparticle assemblies. Mixtures of nanoparticles varying in particle chemistry (or complex refractive indices) and particle size have additional emergent properties that impact the color produced. For such complex multi-component systems, an understanding of assembled structure along with a robust optical modeling tool can empower scientists to perform intensive structure-color relationship studies and fabricate designer materials with tailored color. Here, we demonstrate how we can reconstruct the assembled structure from small-angle scattering measurements using the computational reverse-engineering analysis for scattering experiments (CREASE) method and then use the reconstructed structure in finite-difference time-domain (FDTD) calculations to predict color. We successfully, quantitatively predict experimentally observed color in mixtures containing strongly absorbing melanin nanoparticles and demonstrate the influence of a single layer of segregated nanoparticles on color produced. The versatile computational approach presented in this work is useful for engineering synthetic materials with desired colors without laborious trial and error experiments.

cond-mat.soft↗

arXiv2022

Modeling Structural Colors from Disordered One-Component Colloidal Nanoparticle-based Supraballs using Combined Experimental and Simulation Techniques

Bright, saturated structural colors in birds have inspired synthesis of self-assembled, disordered arrays of assembled nanoparticles with varied particle spacings and refractive indices. However, predicting colors of assembled nanoparticles, and thereby guiding their synthesis, remains challenging due to the effects of multiple scattering and strong absorption. Here, we use a computational approach to first reconstruct the nanoparticles' assembled structures from small-angle scattering measurements and then input the reconstructed structures to a finite-difference time-domain method to predict their color and reflectance. This computational approach is successfully validated by comparing its predictions against experimentally measured reflectance and provides a pathway for reverse engineering colloidal assemblies with desired optical and photothermal properties.

cond-mat.soft↗

arXiv2022

Structural Color Production in Melanin-based Disordered Colloidal Nanoparticle Assemblies in Spherical Confinement

Melanin is a ubiquitous natural pigment that exhibits broadband absorption and high refractive index. Despite its widespread use in structural color production, how the absorbing material, melanin, affects the generated color is unknown. Using a combined molecular dynamics and finite-difference time-domain computational approach, this paper investigates structural color generation in one-component melanin nanoparticle-based supra-assemblies (called supraballs) as well as binary mixtures of melanin and silica (non-absorbing) nanoparticle-based supraballs. Experimentally produced one-component melanin and one-component silica supraballs, with thoroughly characterized primary particle characteristics using neutron scattering, produce reflectance profiles similar to the computational analogues, confirming that the computational approach correctly simulates both absorption and multiple scattering from the self-assembled nanoparticles. These combined approaches demonstrate that melanin's broadband absorption increases the primary reflectance peak wavelength, increases saturation, and decreases lightness factor. In addition, the dispersity of nanoparticle size more strongly influences the optical properties of supraballs than packing fraction, as evidenced by production of a larger range of colors when size dispersity is varied versus packing fraction. For binary melanin and silica supraballs, the chemistry-based stratification allows for more diverse color generation and finer saturation tuning than does the degree of mixing/demixing between the two chemistries.

physics.optics↗

arXiv2020

Transition in the Acid-Base Component of Surface Free Energy of Ice upon the Premelting of its Second Molecular Bilayer

Molecular disordering of the ice surface occurs below the bulk melting temperature of 273 K, termed surface premelting. The top-most molecular layer begins gradually premelting at 200 K, and has been linked to its low coefficient of friction through an increase in molecular mobility. The second molecular bilayer premelts around 257 K, but no study has linked this transition to a change in any macroscopic phenomena. Here, we show that the thermodynamic work of adhesion between polydimethylsiloxane (PDMS) and ice changes abruptly at 257.0 $\pm$ 0.1 K. Surface-sensitive sum frequency generation spectroscopy shows that there are no molecular level changes at the PDMS surface or the ice-PDMS interface near the transition in adhesion, indicating that the transition arises from changes of the ice surface. Using existing contact angle data in the literature, we show that this transition is due to a decrease in the acid-base component of the surface free energy of ice by 17 $\pm$ 2 mJ/m$^2$ at 257.0 $\pm$ 0.1 K. The change in surface energy provides a possible explanation for a variety of unexplained phenomena seen across the literature including ice adhesion, friction, and the morphology of snowflakes.

physics.chem-ph↗