arXiv subjects

Robert W. Stark

Publications and source records attributed to Robert W. Stark.

10 recordsLinked to original sources

arXiv2025

Understanding the swelling behavior of P(DMAA-co-MABP) copolymer in paper-based actuators

As interest in sustainable materials grows, paper is being reimagined as a multifunctional substrate with significant potential for future technologies for innovative, environmentally friendly solutions. This study investigates the swelling behavior and environmental responsiveness of a copolymer, poly(N,N-dimethylacrylamide-co-4-methacryloyloxybenzophenone) (P(DMAA-co-MABP)), when applied to cellulosic paper for use in humidity-sensitive actuators. The copolymer's swelling behavior was characterized using dynamic vapor sorption (DVS) and in-situ atomic force microscopy (AFM). DVS measurements demonstrated that the polymer coating significantly enhances the hygroscopic properties of the paper, while AFM revealed the polymer's fast response to relative humidity (RH) changes, shown by immediate height adjustments, increased adhesion, and decreased stiffness at higher RH levels.Studies on polymer-modified paper-based bilayer actuators demonstrate that incorporating the hydrophilic P(DMAA-co-MABP) results in actuation in response to relative humidity variations between 10% and 90% RH. From these findings, two models were proposed to assess key mechanisms in the swelling behavior: the correlation between the heterogeneity in crosslinking and the polymer swelling behavior, and the correlation between polymer-paper interactions and the hygro-responsive bending behavior. Additionally, thermal analysis was performed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), providing a comprehensive profile of the copolymer's behavior.

cond-mat.mtrl-sci↗

arXiv2025

Confocal Raman microscopy inside sessile multicomponent droplets

Evaporating multicomponent droplets are ubiquitous and appear in a variety of everyday situations and technological applications, including coating, 3D printing, and energy conversion processes. During evaporation, concentration gradients are typically induced, resulting in flows within the droplets. Many of the mechanisms underlying multicomponent droplet evaporation are not fully understood. However, most methods utilize markers that can be surface active and thus affect droplet dynamics. Thus, high-resolution marker-free measurements of concentration gradients in evaporating multicomponent droplets are needed. Raman microscopy can provide such a method. However, as the Raman laser has to undergo a phase transition, it is refracted, leading to a distorted drop contour. In this study, we model refraction in the droplet to better understand the shift in focus leading to geometric distortion. The horizontal and vertical shifts in focus are analyzed, and a modified configuration that enables Raman measurements with a horizontal laser is introduced. For both configurations, the simulated drop shape in the Raman image fits well with the measured shape. While Raman measurements with a vertically incident laser allow the study of the upper part of the droplet, for droplets with large contact angles, horizontal Raman measurements allow for the analysis of the region around the 3-phase contact line. As an example, a concentration map of an evaporating 4.2 $μ$L glycerol/water droplet is presented. The results of this study can be used as a guide for Raman microscopy measurements of droplets. These findings contribute to understanding droplet dynamics during evaporation and provide a basis for developing novel printing, cleaning, and energy conversion technology applications.

physics.flu-dyn↗

arXiv2024

Coalescence of viscoelastic drops on a solid substrate

This study investigated the coalescence of polymer solution drops on the solid substrates. When two drops meet at their contact line on a substrate, the liquid bridge connecting the two drops increases in size with time. The height and radius of the liquid bridge have a power law dependence on time. In the early stage of drop coalescence, the exponents $α$ and $β$ of the power law are influenced by the properties of the polymer solution. We argued that the balance between capillarity and viscoelasticity controls the process, where viscoelasticity must be considered in its full time and shear-rate dependency. As a simple proxy for the processes involved, the ratio of the polymer relaxation time to the viscous time of the drop, the elastocapillary ($Ec$), is instructive. In the vicinity of $Ec= 1$ the exponents showed a minimum and increased to lower and higher values of $Ec$. A similar dependency is observed for the damping timescales of the capillary waves. For high elastocapillary numbers, i.e. high polymer relaxation time, drop coalescence on short timescales behaved as in the low viscosity case. In addition, the bridge profile is influenced by a combination of factors including surface tension, viscosity and polymer stress. In summary, we have shown that drop coalescence is strongly influenced by the viscoelasticity of the drops.

physics.flu-dyn↗

arXiv2023

Conceptualizing flexible papers using cellulose model surfaces and polymer particles

Cellulose, as a naturally abundant and biocompatible material, is still gaining interest due to its high potential for functionalization. This makes cellulose a promising candidate for replacing plastics. Understanding how cellulose interacts with various additives is crucial for creating composite materials with diverse properties, as it is the case for plastics. In addition, the mechanical properties of the composite materials are assumed to be related to the mobility of the additives against the cellulose. Using a well-defined cellulose model surface (CMS), we aim to understand the adsorption and desorption of two polymeric particles (core-shell particles and microgels) to/from the cellulose surface. The nanomechanics of particles and CMS are quantified by indentation measurements with an atomic force microscope (AFM). AFM topography measurements quantified particle adsorption and desorption on the CMS, while peak force AFM measurements determined the force needed to move individual particles. Both particles and the CMS exhibited pH-dependent charge behavior, allowing a tunable interaction between them. Particle adsorption was irreversible and driven by electrostatic forces. In contrast, desorption and particle mobility forces are dominated by structural morphology. In addition, we found that an annealing procedure consisting of swelling/drying cycles significantly increased the adhesion strength of both particles. Using the data, we achieve a deeper understanding of the interaction of cellulose with polymeric particles, with the potential to advance the development of functional materials and contribute to various fields, including smart packaging, sensors, and biomedical applications.

cond-mat.soft↗

arXiv2021

Concentration Gradients in Evaporating Binary Droplets Probed by Spatially Resolved Raman and NMR Spectroscopy

Understanding the evaporation process of binary sessile droplets is essential for optimizing various technical processes, such as inkjet printing or heat transfer. Liquid mixtures whose evaporation and wetting properties may differ significantly from those of pure liquids are particularly interesting. Concentration gradients may occur in these binary droplets. The challenge is to measure concentration gradients without affecting the evaporation process. Here, spectroscopic methods with spatial resolution can discriminate between the components of a liquid mixture. We show that confocal Raman microscopy and spatially resolved nuclear magnetic resonance (NMR) spectroscopy can be used as complementary methods to measure concentration gradients in evaporating 1-butanol/1-hexanol droplets on a hydrophobic surface. Deuterating one of the liquids allows analysis of the local composition through the comparison of the intensities of the CH and CD stretching bands in Raman spectra. Spatially resolved NMR spectroscopy is used to measure the composition at different positions of the droplet. Confocal Raman and spatially resolved NMR experiments show the presence of a vertical concentration gradient as the 1-butanol/1-hexanol droplet evaporates.

physics.app-ph↗

arXiv2021

Nanomechanical subsurface characterisation of cellulosic fibres

The mechanical properties of single fibres are highly important in the paper production process to produce and adjust properties for the favoured fields of application. The description of mechanical properties is usually characterised via linearized assumptions and is not resolved locally or spatially in three dimensions. In tensile tests or nanoindentation experiments on cellulosic fibres, only one mechanical parameter, such as elastic modulus or hardness, is usually obtained. To obtain a more detailed mechanical picture of the fibre, it is crucial to determine mechanical properties in depth. To this end, we discuss an atomic force microscopy-based approach to examine the local stiffness as a function of indentation depth via static force-distance curves. This method has been applied to linter fibres (extracted from a finished paper sheet) as well as to natural raw cotton fibres to better understand the influence of the pulp treatment process in paper production on the mechanical properties. Both types of fibres were characterised in dry and wet conditions with respect to alterations in their mechanical properties. Subsurface imaging revealed which wall in the fibre structure protects the fibre against mechanical loading. Via a combined 3D display, a spatially resolved mechanical map of the fibre interior near the surface can be established. Additionally, we labelled fibres with carbohydrate binding modules tagged with fluorescent proteins to compare the AFM results with fluorescence confocal laser scanning microscopy imaging. Nanomechanical subsurface imaging is thus a tool to better understand the mechanical behaviour of cellulosic fibres, which have a complex, hierarchical structure.

physics.app-ph↗

arXiv2020

Mapping Humidity-dependent Mechanical Properties of a Single Cellulose Fibre

Modelling of single cellulose fibres is usually performed by assuming homogenous properties, such as strength and Young s modulus, for the whole fibre. Additionally, the inhomogeneity in size and swelling behaviour along the fibre is often disregarded. For better numerical models, a more detailed characterization of the fibre is required. Herein, we report a method based on atomic force microscopy to map these properties along the fibre. A fibre was mechanically characterized by static colloidal probe AFM measurements along the fibre axis. Thus, the contact stress and strain at each loading point can be extracted. Stress strain curves can be obtained along the fibre. Additionally, mechanical properties such as adhesion or dissipation can be mapped. The inhomogeneous swelling behaviour was recorded via confocal laser scanning microscopy along the fibre. Scanning electron microscopy measurements revealed the local macroscopic fibril orientation and provided an overview of the fibre topology. By combining these data, regions along the fibre with higher adhesion, dissipation, bending ability and strain or differences in the contact stress when increasing the relative humidity could be identified. This combined approach allows for one to obtain a detailed picture of the mechanical properties of single fibres.

physics.app-ph↗

arXiv2020

Nanomechanical characterisation of a water-repelling terpolymer coating of cellulosic fibres

Polymer coatings on cellulosic fibres are widely used to enhance the natural fibre properties by improving, for example, the hydrophobicity and wet strength. Here, we investigate the effects of a terpolymer P(S-co-MABP-co-PyMA) coating on cotton linters and eucalyptus fibres to improve the resistance of cellulose fibres against wetness. Coated and uncoated fibres were characterised by using scanning electron microscopy, contact angle measurements, Raman spectroscopy and atomic force microscopy with the objective of correlating macroscopic properties such as the hydrophobicity of the fleece with microscopic properties such as the coating distribution and local nanomechanics. The scanning electron and fluorescence microscopy results revealed the distribution of the coating on the paper fleeces and fibres. Contact angle measurements proved the hydrophobic character of the coated fleece, which was also confirmed by Raman spectroscopy measurements that investigated the water uptake in single fibres. The water uptake also induced a change in the local mechanical properties, as measured by atomic force microscopy. These results verify the basic functionality of the hydrophobic coating on fibres and paper fleeces but call into question the homogeneity of the coating.

physics.app-ph↗

arXiv2019

Zero Field Assembly of Long Magnetic Dipolar Chains in 2D Polymer Nanocomposite Films

The existence of magnetic dipolar nanoparticle chains at zero field has been predicted theoretically for decades, but these structures are rarely observed experimentally. A prerequisite is a permanent magnetic moment on the particles forming the chain. Here we report on the observation of magnetic dipolar chains of spherical iron oxide nanoparticles with a diameter of \SI{12.8}{\nano\meter}. The nanoparticles are embedded in an ultrathin polymer film. Due to the high viscosity of the polymer matrix, the dominating aggregation mechanism is driven by dipolar interactions. Smaller iron oxide nanoparticles (\SI{9.4}{\nano\meter}) show no permanent magnetic moment and do not form chains but compact aggregates. Mixed monolayers of different iron oxide nanoparticles and polymer at the air-water interface are characterized by Langmuir isotherms and in-situ X-ray reflectometry (XRR). The combination of the particles with a polymer leads to a stable polymer nanocomposite film at the air-water interface. XRR experiments show that nanoparticles are immersed in a thin polymer matrix of \SI{3}{\nano\meter}. Using atomic force microscopy (AFM) on Langmuir-Blodgett films, we measure the lateral distribution of particles in the film. An analysis of single structures within transferred films results in fractal dimensions that are in excellent agreement with 2D simulations.

cond-mat.soft↗

arXiv2005

Dynamic Force Spectroscopy: Looking at the Total Harmonic Distortion

Tapping mode atomic force microscopy is a standard technique for inspection and analysis at the nanometer scale. The understanding of the non-linear dynamics of the system due to the tip sample interaction is an important prerequisite for a correct interpretation data acquired by dynamic AFM. Here, the system response in tapping-mode atomic force microscope (AFM) simulated numerically. In the computer model the AFM microcantilever is treated as a distributed parameter system. With this multiple-degree-of-freedom (MDOF) approach the the total harmonic distortion in dynamic AFM spectroscopy is simulated.

physics.ins-det↗