Spreading of Water Droplets on Cellulose-Based Papers: the Effect of Back-Surface Coating.

Regulation of wetting and spreading of liquid on porous material plays an important role in a variety of applications, such as waterproofing, anti-icing, antioxidation, self-cleaning, etc. In this work, we reveal the role of back-surface coating with superhydrophobic nanoparticles in controlling the spreading of water droplets on cellulose-based papers. A layer of superhydrophobic polydivinylbenzene (PDVB) nanoparticles is spin-coated on the back surface of different types of papers. The spreading of a water droplet on the top, uncoated surface is dependent on the size of the PDVB nanoparticles in the coating. Using a relationship derived from Darcy's law, we observe that the energy barrier for the spreading of water droplets on three types of papers (heavy-weight, light-weight, and slight-weight papers) decreases with the decrease of the nanoparticle size in the back-surface coating. The spreading of the water droplet is dependent on the porous structure, permeability, and compressibility of the papers. The method presented in this work provides a feasible approach to use the back-surface coating to control the wettability of papers.

Kinetics for the Slip Motion of Ripple Dislocations on Gold-Coated Poly(dimethylsiloxane).

Understanding the evolution of the defects in surface patterns is of practical importance to improve the performance and structural durability of the pattern-based micro- and nanodevices. In this work, we investigate the effects of temperature, compressive strain, and relative direction of the compression to the prestretch on the slip motion of ripple dislocations formed on the surface of gold-coated poly(dimethylsiloxane) films. Applying compression in the direction parallel to the direction of prestretch cannot cause the slip motion of the ripple dislocations. The initial velocity of the slip motion of the ripple dislocations increases with the increases in temperature and compressive strain. The temperature dependence of the ratios of the configuration force to the viscous coefficient and the viscous coefficient to the effective mass of the ripple dislocations follows the Arrhenius equation.

Mechanical creep of tea leaves.

BACKGROUND: Tea processing involves fermentation, withering, steaming or pan-firing, rolling, baking, and drying. Some of these steps are performed at a high temperatures. At such temperatures the creep of the tea leaves plays an important role in the quality of tea. In materials science, creep is the tendency of a tea leaf to move slowly or defom permanently under a constant load. There has been much research on the mechanical properties of the outmost cuticular layer of leaves but there are few reports addressing the mechanical properties of whole leaves. RESULTS: We cut tea leaf into specimen of dog-bone shape and measure the time-dependent creep deformation using a dynamic mechnical analyzer. Three different tea leaves grown in Taiwan were examined. The nonlinear Burgers model is proposed to describe the creep deformation of the tea leaves. CONCLUSIONS: The creep of the tea leaves consists of primary and steady states, and the creep deformation is accurately described by the Kelvin representation of the nonlinear Burgers model. The viscosities in the primary stages satisfied the Arrhenius equation, and the activation energies were determined. The stress exponents for the creep of the tea leaves were less than unity. The Maxwell representation of the Burgers model is mathematically equivalent to the Kelvin representation of the Burgers model and can also be used to explain the creep of tea leaves. © 2020 Society of Chemical Industry.

Growth of Polystyrene Pillars in Electric Field.

Surface patterning on polymer films, which is a self-assembly process under the action of external and/or internal impetus, has a variety of applications, including drug delivery and flexible electronics. In this work, we study the growth of polystyrene pillars in the electric field for different combinations of annealing temperature, film thickness, and electrode separation (electric field intensity). There are five stages for the growth of the polystyrene pillars for all the configurations used in this work, including a nucleation stage, a linear growth stage, an acceleration stage in the pillar length prior to the contact between the top surface of a pillar and the upper electrode, a radial growth stage after the contact, and a stationary stage without further growth of the pillar. In the linear growth stage, there exist linear relationships between the pillar length and the annealing time and between the square of the pillar diameter and the annealing time. The activation energies for the rate processes controlling the radial growth and the length growth in the linear growth stage are 30.2 and 25.3 kJ/mol, respectively. There are two rate processes controlling the radial growth of the pillars: one is the field-induced flow of polymer through the polymer film to the roots of pillars and the other is the coalescence of pillars. The activation energy for the coalescence is 16.5 kJ/mol. The results obtained in this work offer a practical route to control the geometrical dimensions of polymer pillars through the processing parameters.

Brownian motion and Einstein relation for migration of coffee particles in coffee suspensions.

BACKGROUND: The migration of particles in a liquid plays an important role in determining the stability of the corresponding suspension. The preparation of a cup of coffee involves the migration/dispersion of coffee particles in aqueous solutions. We investigate the Brownian motion of coffee particles in coffee suspensions at different temperatures for three different coffee beans via the migration of the coffee particles of the coffee suspensions in water. RESULTS: The activation energies for the Brownian motion of the coffee particles are in the range 23.5-32.0 kJ mol-1 , relatively independent of the size of the coffee particles used in the present study. The viscosities of the coffee suspensions are measured as a function of temperature and then used to correlate with the gradient-diffusion coefficient for the Brownian motion of the coffee particles. The activation energies of the rate process controlling the viscous flow of the coffee suspensions are in the range 12.7-14.1 kJ mol-1 . CONCLUSION: The correlation between the viscosity and gradient-diffusion coefficient of the coffee suspensions generally follows the Einstein relation. A temperature dependence exists for the viscosity and gradient-diffusion coefficient of the coffee suspensions, which can be used to understand the brewing of coffee at high temperatures with respect to product refinement. © 2019 Society of Chemical Industry.

Effect of graphene on the absorption of methanol and crack healing in poly(methyl methacrylate)-based composites.

This work is focused on the mass transport of methanol and the methanol-assisted crack healing in poly(methyl methacrylate) (PMMA)-graphene composites at different temperatures. The effect of the fraction of graphene on the mass transport of methanol and the methanol-assisted crack healing is also studied. The experimental results reveal that adding graphene to the PMMA matrix increases the resistance to the migration/diffusion of methanol and polymer chains in the PMMA matrix, and the absorption of methanol follows anomalous diffusion. The activation energies for the case I transport and case II transport in the PMMA-graphene composites are relatively independent of the fraction of graphene, and are larger than the corresponding ones in pure PMMA. Increasing the healing time and healing temperature allows for more polymer chains to migrate/diffuse across fractured surfaces, leading to the increase in the fracture strength of the crack-healed PMMA-graphene composites.

Absorption behavior of poly(methyl methacrylate)-multiwalled carbon nanotube composites: effects of UV irradiation.

Understanding the effects of carbon nanotubes (CNTs) and ultraviolet (UV) irradiation on solvent transport in polymers is of practical importance for the applications of polymer-CNT composites in electronics and photonics. The transport behavior of methanol in poly(methyl methacrylate)-multiwalled carbon nanotube (PMMA-MWCNT) composites with and without UV light irradiation has been studied. The anomalous transport has been investigated as a function of the weight percentage of MWCNTs and UV dose in the temperature range of 30-50 °C. The anomalous transport consists of Case I (controlled by concentration gradient) and Case II (controlled by stress relaxation) transport; both UV irradiation and the addition of MWCNTs in PMMA enhance the transport of methanol. The activation energies for Case I and Case II transport decrease with the increase of UV dose for the PMMA-MWCNT plates with the same weight percentage of MWCNTs. Without UV irradiation, the activation energy for Case I transport of methanol decreases with the increase of the weight percentage of MWCNTs, and the activation energy for Case II transport increases with the increase of the weight percentage of MWCNTs.

Kinetics of Field-Induced Surface Patterns on PMMA.

A simple model was developed to analyze the growth of a liquid pillar under the action of an electric field between two parallel electrodes. A quadratic relationship between time and the diameter of the pillar was obtained. The diameter of the pillar increases with time. Large electric field assists the growth of the liquid pillar, while a liquid with a large viscosity hinders the growth of the liquid pillar. The field-induced formation and growth of PMMA pillars on PMMA films were observed using the configuration of a parallel capacitor. Pillars of larger sizes and smaller densities were formed on thicker PMMA films than on thinner PMMA films. The root-mean-square ( https://en.wikipedia.org/wiki/Root_mean_square ) diameter of the pillars increases with the increase of the annealing time and annealing temperature. The growth behavior of the pillars can be described by an Arrhenius relation with an activation energy of 24.4 kJ/mol, suggesting that the growth of the pillars is controlled by a thermal activation process.

Thermal Degradation of Vegetable Oils.

Vegetable oils provide lipids and nutrition and provide foods with a desirable flavor, color, and crispy texture when used to prepare fried foods. However, the oil quality is degraded at elevated temperatures, and thus must be examined frequently because of the damage to human health. In this study, sunflower, soybean, olive, and canola oils were examined, and their properties were measured periodically at different elevated temperatures. The unsaturated triglyceride in oils reacted with the environmental oxygen or water vapor significantly changes in optical absorbance, viscosity, electrical impedance, and acid value. We used defect kinetics to analyze the evolution of these oil properties at elevated temperatures. The optical absorbance, viscosity, and electrical impedance follow the second-order, first-order, and zeroth-order kinetics, respectively. The rate constants of the above kinetics satisfy the Arrhenius equation. Olive oil has the lowest rate of color center and dynamic viscosity among the four oils, with the smallest pre-exponential factor and the largest activation energy, respectively. The rate constants of acid reaction also satisfy the Arrhenius equation. The activation energies of the polar compound and acid reaction are almost the same, respectively, implying that the rate constant is controlled by a pre-exponential factor if four oils are compared. Olive oil has the largest rate constant of acid reaction among the four oils, with the lowest pre-exponential factor.

Thermo-Mechanical and Creep Behaviour of Polylactic Acid/Thermoplastic Polyurethane Blends.

There is a great need to develop biodegradable thermoplastics for a variety of applications in a wide range of temperatures. In this work, we prepare polymer blends from polylactic acid (PLA) and thermoplastic polyurethane (TPU) via a melting blend method at 200 °C and study the creep deformation of the PLA/TPU blends in a temperature range of 10 to 40 °C with the focus on transient and steady-state creep. The stress exponent for the power law description of the steady state creep of PLA/TPU blends decreases linearly with the increase of the mass fraction of TPU from 1.73 for the PLA to 1.17 for the TPU. The activation energies of the rate processes for the steady-state creep and transient creep decrease linearly with the increase of the mass fraction of TPU from 97.7 ± 3.9 kJ/mol and 59.4 ± 2.9 kJ/mol for the PLA to 26.3 ± 1.3 kJ/mol and 25.4 ± 1.7 kJ/mol for the TPU, respectively. These linearly decreasing trends can be attributed to the weak interaction between the PLA and the TPU. The creep deformation of the PLA/TPU blends consists of the contributions of individual PLA and TPU.

Stress Relaxation Behavior of Poly(Methyl Methacrylate)/Graphene Composites: Ultraviolet Irradiation.

The graphene/poly (methyl methacrylate) (PMMA) composites are a promising candidate for electronic, optoelectrical, and environmental applications. Understanding the mechanical degradation of PMMA-based materials is of practical importance in improving the reliability and lifespan of the associated structures and systems. In this study, we investigate the effects of functionalized graphene (FG) and UV irradiation on the stress-relaxation of PMMA. Uniaxial tensile and stress -relaxation tests are performed to evaluate the mechanical properties of the composites. The mechanical strength and elongation at the break increase with the graphene concentration but decrease with the increase of the irradiation dose. Raman spectroscopy and intrinsic viscosity measurement are applied to examine the root cause of the degradation in the composites. UV irradiation leads to polymer chain scission and loss of molecular weight. The Kelvin representation of the standard linear solid model (SLSM) is used to describe the stress-relaxation curves of the composites. The value of the elastic modulus in the Kelvin element decreases with the increase in temperature. The viscosity follows the Arrhenius equation. The activation energy of viscosity increases with the increasing FGs concentration because the FGs hinder the chain motion of PMMA. However, UV irradiation makes chain scission of PMMA/FGs composite so that the polymer chain moves more easily and the activation energy of stress relaxation lowers. The steady-state stress follows the van 't Hoff equation that stress relaxation is an exothermal deformation process. Although Maxwell's representation of SLSM is mathematically identical to the Kelvin representation of SLSM, the former cannot interpret the stress-relaxation behavior of PMMA/FGs composite, which is against the concept of Young's modulus as a decreasing temperature function.

Time-dependent deformation of artificial muscles based on Nylon 6.

Considering the potential applications of the Nylon 6 with thermal-induced deformation, we studied the creep deformation of non-twisted Nylon 6 wires and Nylon 6 artificial muscles as functions of annealing temperature. For comparison, we also studied the creep deformation of chicken muscle fibers in a temperature range of 20 to 35 °C. The experimental results showed that we could use the standard linear viscoelastic model to describe the creep deformation of the chicken muscle fibers, the non-twisted Nylon 6 wires, and the Nylon 6 artificial muscles. A simple method was developed to calculate the mechanical (elastic) constants and viscous resistance coefficient (viscosity) of the three different materials. The activation energy for the creep deformation of the chicken muscle fibers in the temperature of 20 to 35 °C was 18.79 kJ/mol. For the non-twisted Nylon 6 wires, the activation energy for the creep deformation was generally larger than that of the chicken muscle fibers, and was dependent on the annealing temperature. For the Nylon 6 artificial muscles, the activation energy for the creep deformation was smaller than that of the chicken muscle fibers.

Caffeine Extraction from Raw and Roasted Coffee Beans.

Coffee is a stimulant, psychoactive, popular daily beverage, and its caffeine affects human physiological health and behavior. These important issues prompted us to study caffeine extraction from both the raw and roasted coffee beans of 3 types at different temperatures. A hemispheric model is developed to simulate the extraction process of the caffeine from the coffee beans of hemisphere is proposed. The experimental data are in good agreement with the predicted model. The effective diffusivities of caffeine in both the raw and roasted beans increase with temperature in all 3 types. An incubation period, decreasing with increasing temperature, is observed in all samples studied. Caffeine extraction in roasted beans is more rapid than that for the raw beans and the time difference is significant at low temperatures. In both the raw and roasted samples, caffeine diffusion in the raw beans and the incubation behavior are thermally activated processes. Single activation energies are obtained for diffusion within the extraction temperature range for all beans tested with the exception of one type of the coffee beans, Mandheling, which exhibits 2 activation energies in raw samples. The surface energies of the epidermis of the raw beans and roasted beans obtained from the contact angle measurements are used to interpret the difference of incubation periods. PRACTICAL APPLICATION: This study has a potential application to the decaffeinated coffee industry.Caffeine affects human physiological health and behavior so that caffeine extraction from coffee beans of different types at different temperatures is important for product refining and customers.

Surface wrinkling of an elastic film: effect of residual surface stress.

The effect of residual surface compression on the surface evolution of solid thin films was analyzed. Analytical relation was derived among the apparent surface stress, the spatial frequency of the surface modulation, and the film thickness. Using this relationship, we calculated the dependence of the apparent surface stress on the film thickness from the experimental results of the polymer resist coated on glass slide. The magnitude of the apparent surface stress decreased with the reduction in the film thickness, and it approached a constant of 0.46 kN/m as the thickness of the films approached zero. The result is possibly applied to nanoimprint technology.