Lateral surface nonuniformities in drying latex films

Effect of film thickness and particle size on cracking stresses in drying latex films

Using an experimental setup which determines the turbidity of the sample and the efficiency of Förster resonance energy transfer (FRET) at the same time, we have correlated the particle deformation kinetics in a drying latex film, quantified by light scattering with the kinetics of polymer interdiffusion. Interdiffusion was quantified making use of energy transfer (FRET) between donor molecules and acceptor molecules, bound to polymer chains on different particles. When the chains cross the interparticle boundaries, the rate of energy transfer increases. The latex was prepared by miniemulsion polymerization. The amount of emulsifier employed during polymerization had a pronounced effect on the relative timing of interdiffusion and particle deformation. Increasing the amount of emulsifier delayed the onset of interdiffusion relative to the time when the film became transparent. This is mostly the consequence of a size effect, as opposed to surfactant acting as a barrier for transport.

Using lubrication theory, drying processes of sessile colloidal droplets on a solid substrate are studied. A simple model is proposed to describe temporal dynamics of both the shape of the drop and the volume fraction of the colloidal particles inside the drop. The concentration dependence of the viscosity is taken into account. It is shown that the final shapes of the drops depend on both the initial volume fraction of the colloidal particles and the capillary number. The results of our simulations are in a reasonable agreement with the published experimental data. Computations for the drops of aqueous solution of human serum albumin are presented.

A multi-scale finite element method is developed to simulate the irradiation process and postirradiation uniaxial tensile tests for metal-matrix composite fuels with representative volume elements (RVEs). The simulations of irradiation process are implemented under a wide range of burnup levels, with the irradiation effects on the mechanical constitutive relations of fuel particles and matrix taken into account comprehensively. The simulation results for the macroscopic postirradiation true stress/strain curves are obtained, excluding the irradiation-induced macroscopic deformations. The effects of particle fission density, temperature, and initial particle volume fraction are investigated and analyzed. The research results indicate that 1) a quasi-elastic stage appears during the postirradiation tension, which is mainly induced by the creation of high residual compressive stresses in the particles and matrix after irradiation; 2) with the increase of effective strains, new plastic deformations increase in the particles and matrix to result in the macroscale plastic stage; 3) the macroscale irradiation softening and hardening phenomena appear, which mainly stem from the weakened deformation resistance by the irradiation-induced plastic deformations in the matrix, the enlarged particle volume fraction after irradiation, and the irradiation hardening effects of metal matrix.

The interactions between an incident shock and a moderately dense particle curtain are simulated with the Eulerian–Lagrangian method. A customized solver based on OpenFOAM is extended with an improved drag model and collision model and then validated against two benchmark experiments. The results show that the collision model has a limited impact on curtain morphology compared with the improved drag model. In this work, parametric studies are performed considering different particle sizes, volume fractions, and curtain thicknesses. Smaller particle sizes and larger volume fractions lead to stronger reflected shock and weaker transmitted shock. Attention is paid to the particle collision effects on the curtain evolution behaviors. According to our results, for the mono-dispersed particle curtain, the collision effects on curtain front behaviors are small, even when the initial particle volume fraction is as high as 20%. This is due to the positive velocity gradient across the curtain after the shock wave passage, leading to the faster motion of downstream particles than the upstream ones, and hence, no collision occurs. For the bi-dispersed particle curtain, the collision effects become important in the mixing region of different-size particles. Collisions decelerate small particles while accelerating large ones and cause velocity scattering. Moreover, increasing the bi-dispersed curtain thickness leads to multiple collision force peaks, which is the result of the delayed separation of different particle groups. Our results indicate that the collision model may be unnecessary to predict curtain fronts in mono-dispersed particles, but in bi-dispersed particles, the collision effects are important and, therefore, must be modeled.

Colloidal particles in a tiny drying droplet are forced to assemble due to attractive capillary forces. Jamming of the particles throughout the droplet remains either isotropic or anisotropic depending upon the drying kinetics and the physicochemical environment. In this work, we explore the dynamical evolution of such an assembly process across a single evaporative droplet by in situ scanning small-angle scattering using a micro-focused X-ray beam at the synchrotron source. A methodology has been elucidated to differentiate quantitatively between the isotropic and the anisotropic jamming process. Switching of jamming behaviour depending on the initial particle volume fraction in the droplet has been demonstrated. Three distinct stages of assembly, associated with droplet shrinkage, have been revealed even during isotropic jamming. This is in contrast to the drying of a pure liquid droplet under diffusion limited evaporation. It has been established that such in situ scattering measurements can also be used to estimate the temporal evolutions of the viscosity of a drying suspension as well as the diffusivity of nanoparticles in a droplet.

The change of minimum film formation temperature (MFFT) with time was studied utilizing a temperature gradient bar. Fitting the data to a theory which assumed that particles deform due to the action of the polymer-air surface tension, the glass transition temperature, Tg, of the latex was predicted. This Tg was compared to the value obtained by differential scanning calorimetry (DSC) and good agreement was observed between the two measurements.

We report on spatially resolved measurements of the mechanical stress in drying polymer films. The technique is based on the deflection of a flexible membrane serving as the substrate. Assuming that the lateral tension of the membrane is the main source of its resistance to deformation, one can show that the local surface stress sigma f (x,y) is proportional to the vertical displacement of the membrane u z(x,y). The membrane distortion was determined by optical means. Measurements taken on drying latex dispersions revealed a maximum of film stress at the rim. The heterogeneous stress distribution often persisted after the film had become dry.

Latex dispersions with different glass-transition temperatures have been dried in Petri dishes over a range of temperatures. The crack spacing is reported and is observed to increase as the temperature approaches the glass-transition temperature of the latex. The data for crack spacing over a range of temperatures is collapsed to a single curve by adapting a scaling for the crack spacing of hard particles. The increase in temperature requires an account to be made for particle deformation and, hence, an increase in particle volume fraction, as well as the temperature dependence of the evaporation rate. The importance of measuring the stress at which films crack is highlighted.

Decoupling of viscous dissipation and damage dissipation in particulate-reinforced polymeric materials

Aqueous polymer colloids known as latexes are widely used in coating applications. Multicomponent latexes comprised of two incompatible polymeric species organized into a core-shell particle morphology are a promising system for self-stratifying coatings that spontaneously partition into multiple layers, thereby yielding complex structured coatings requiring only a single application step. Developing new materials for self-stratifying coatings requires a clear understanding of the thermodynamic and kinetic properties governing phase separation and polymeric species transport. In this work, we study phase separation and self-stratification in polymer films based on multicomponent acrylic (shell) and acrylic-silicone (core) latex particles. Our results show that the molecular weight of the shell polymer and heat aging conditions of the film critically determine the underlying transport phenomena, which ultimately controls phase separation in the film. Unentangled shell polymers result in efficient phase separation within hours with heat aging at reasonable temperatures, whereas entangled shell polymers effectively inhibit phase separation even under extensive heat aging conditions over a period of months due to kinetic limitations. Transmission electron microscopy is used to track morphological changes as a function of thermal aging. Interestingly, our results show that the rheological properties of the latex films are highly sensitive to morphology, and linear shear rheology is used to understand morphological changes. Overall, these results highlight the importance of bulk rheology as a simple and effective tool for understanding changes in morphology in multicomponent latex films.

Recent experimental advances have revealed that the mean free path (mfp) of phonons contributing significantly to thermal transport in crystalline semiconductors can be several microns long. Almost all of these experiments are based on bulk and thin film materials and use techniques that are not directly applicable to nanowires. By developing a process with which we could fabricate multiple electrically contacted and suspended segments on individual heavily doped smooth Silicon nanowires, we measured phonon transport across varying length scales using a DC self-heating technique. Our measurements show that diffusive thermal transport is still valid across O(100) nm length scales, supporting the diffuse nature of phonon-boundary scattering even on smooth nanowire surfaces. Our work also showcases the self-heating technique as an important alternative to the thermal bridge technique to measure phonon transport across short length scales relevant to mapping the phonon mfp spectrum in nanowires.

Preparation of interconnected poly(ε-caprolactone) porous scaffolds by a combination of polymer and salt particulate leaching

<p>Wind-blown sand dunes emerge due the linear instability of a flat sedimentary bed. This instability has been studied in experiments and numerical models but rarely in the field, because of the large time and length scales involved. Here we examine dune formation at the upwind margin of the White Sands Dune Field in New Mexico (USA), using 4 years of lidar topographic data to follow the spatial and temporal development of incipient dunes. Data quantify dune wavelength, growth rate, and propagation velocity and also the characteristic length scale associated with the growth process. We show that all these measurements are in quantitative agreement with predictions from linear stability analysis. This validation makes it possible to use the theory to reliably interpret dune-pattern characteristics and provide quantitative constraints on associated wind regimes and sediment properties, where direct local measurements are not available or feasible.</p><p><strong>Reference</strong>: Gadal et al., Geophys. Res. Lett. <strong>47</strong>, e2020GL088919 (2020).</p>

Aims. We study the stability properties of relativistic magnetized astrophysical jets in the linear regime. We consider cylindrical cold jet configurations with constant Lorentz factor and constant density profiles across the jet. We are interested in probing the properties of the instabilities and identifying the physical quantities that affect the stability profile of the outflows. Methods. We conducted a linear stability analysis on the unperturbed outflow configurations we are interested in. We focus on the unstable branches, which can disrupt the initial outflow. We proceeded with a parametric study regarding the Lorentz factor, the ratio of the rest mass density of the jet to that of the environment, the magnetization, and the ratio of the poloidal component of the magnetic field to its toroidal counterpart measured on the boundary of the jet. We also consider two choices for the pressure of the environment, either thermal or magnetic, and check if this choice affects the results. Additionally, we applied a WKBJ method at the radius of the jet in order to study the local stability properties. Finally, we adapted the jet configuration in Cartesian geometry and compared the planar flow results with the results of the cylindrical counterpart. Results. While investigating the stability properties of the configurations, we observed the existence of a specific solution branch, which showcases the growth timescale of the instability comparable to the light crossing time of the jet radius. Our analysis focuses on this solution. All of the quantities considered for the parametric study affect the behavior of the mode while the magnetized environments seem to hinder its development compared to the hydrodynamic equivalent. Also, our analysis of the eigenfunctions of the system alongside the WKBJ results show that the mode develops in a very narrow layer near the boundary of the jet, establishing the notion of locality for the specific solution. The results indicate that the mode is a relativistic generalization of the Kelvin-Helmholtz instability. We compare this mode with the corresponding solution in Cartesian geometry and define the prerequisites for the Cartesian Kelvin-Helmholtz to successfully approximate the cylindrical counterpart. Conclusions. We identify the Kelvin-Helmholtz instability for a cold nonrotating relativistic jet carrying a helical magnetic field. Our parametric study reveals the important physical quantities that affect the stability profile of the outflow and their respective value ranges for which the instability is active. The Kelvin-Helmholtz mode and its stability properties are characterized by the locality of the solutions, the value of the angle between the magnetic field and the wavevector, the linear dependence between the mode’s growth rate and the wavevector, and finally the stabilization of the mode for flows that are ultrafast magnetosonic. The cylindrical mode can be approximated successfully by the Cartesian Kelvin-Helmholtz instability whenever certain length scales are much larger than the jet radius.