D. van der Meer

Crystal melting and its kinetics on poly(ethylene oxide) by in situ atomic force microscopy

The process of melting in poly(ethylene oxide) (PEO) is followed in real-time at elevated temperatures by atomic force microscopy (AFM) using a simple hot stage apparatus. AFM imaging of the morphology above the onset of melting revealed the dynamics of a complex melting process. The observed melting behavior of PEO is associated with the existence of separate dominant and subsidiary morphological entities. The morphological observations revealed that the melting process is not explained by a mechanism of crystal reorganization (melting–recrystallization–remelting or crystal thickening. The kinetic data shows that the crystal dimensions decrease proportional to time indicating a nucleation controlled melting process. The crystals melt instantaneously on heating and reveal a spread in the rates of melting of the radial {120} faces. This variation in rate of retrogression of the crystals is assumed to be related to a lamellar thickness distribution of the melt grown crystals.

Splash wave and crown breakup after disc impact on a liquid surface

In this paper we analyse the impact of a circular disc on a free surface using experiments, potential flow numerical simulations and theory. We focus our attention both on the study of the generation and possible breakup of the splash wave created after the impact and on the calculation of the force on the disc. We have experimentally found that drops are only ejected from the rim located at the top part of the splash – giving rise to what is known as the crown splash – if the impact Weber number exceeds a threshold value Wecrit?140. We explain this threshold by defining a local Bond number Botip based on the rim deceleration and its radius of curvature, with which we show using both numerical simulations and experiments that a crown splash only occurs when Botip?1, revealing that the rim disrupts due to a Rayleigh–Taylor instability. Neglecting the effect of air, we show that the flow in the region close to the disc edge possesses a Weber-number-dependent self-similar structure for every Weber number. From this we demonstrate that Botip?We, explaining both why the transition to crown splash can be characterized in terms of the impact Weber number and why this transition occurs for Wecrit?140. Next, including the effect of air, we have developed a theory which predicts the time-varying thickness of the very thin air cushion that is entrapped between the impacting solid and the liquid. Our analysis reveals that gas critically affects the velocity of propagation of the splash wave as well as the time-varying force on the disc, FD. The existence of the air layer also limits the range of times in which the self-similar solution is valid and, accordingly, the maximum deceleration experienced by the liquid rim, that sets the length scale of the splash drops ejected when We>Wecrit.

Effects of contact time, humidity, and surface roughness on the adhesion hysteresis of polydimethylsiloxane

Dynamic contact mechanics experiments have been performed on small polydimethylsiloxane (PDMS) lenses and several substrates in both ambient air and in dry nitrogen. The experimental results are analyzed with the Johnson-Kendall-Roberts theory. While the theory adequately describes the approach data, it is unable to account for the large hysteresis observed upon retraction. Adhesion hysteresis is shown to scale with the roughness of the substrate, the hydrophilicty of the substrate, the time of contact, and the ambient humidity. The experimental results also demonstrate that this method is sensitive to changes in the surface energy of the substrate. The cumulative adhesion hysteresis is quantified and is shown to be largest for rough, hydrophilic substrates in relatively high humidity and smallest for smooth substrates in dry nitrogen. The origin of the hysteresis is analyzed by considering favorable interfacial bonding resulting from water-mediated bonding between the substrate and oxygen atoms in the PDMS backbone or other polar species on the polymer surface. Capillary forces are also postulated to contribute to the cumulative adhesion hysteresis.

The effect of liquid viscosity on bubble pinch-off

The collapse stage of an air bubble immersed in a stagnant viscous liquid is experimentally and theoretically investigated, focusing on the effect of liquid viscosity on the final instants previous to pinch-off. Our experiments are consistent with recent investigations, and at the same time highlight several important limitations of previous works. In particular, it is shown that the use of a power law to describe the collapse dynamics of the bubble is not appropriate in an intermediate range of liquid viscosities, for which a transition from an inviscid to a fully viscous pinch-off takes place. Under these conditions, the instantaneous exponent α(τ) varies during a single pinch-off event from the typical values of inviscid collapse, α ≃ 0.58, to the value corresponding to a fully viscous dynamics, α ≃ 1. Consequently, the effective exponent of the power law is not correctly defined in these cases. However, as in the work of Bolanos-Jimenez et al. [Phys. Fluids 20, 112104 (2008) ], we show that the pinch-off process can be accurately described by the use of a pair of Rayleigh-like differential equations for the time evolution of the minimum radius, R0, and half the axial curvature evaluated at the minimum radius, r1. In particular, the theoretical model is able to describe the smooth transition which takes place from inviscid to viscous-dominated pinch-off in liquids of intermediate viscosity, 10 ≤ μ ≤ 100 cP, and accounts for the fact that the axial curvature remains constant when the local Reynolds number becomes small enough, in close agreement with our experimental measurements.

ON THE DEPENDENCE OF IMPACT BEHAVIOR ON THE CRYSTALLINE MORPHOLOGY IN POLYPROPYLENES

The correlation between impact resistance and crystalline morphology was studied on specimens prepared from three different polypropylene (PP) homo- and copolymers, obtained by injection molding. The crystalline morphology was varied using three different nucleating agents. Linear elastic fracture mechanics was applied for the description of fracture resistance. The results indicate that morphological characteristics strongly influence the mechanical performance of the polymers. According to Youngs theory for yielding, the resistance to crack initiation, represented by K Ic, correlates with the thickness of the lamellae. This suggests that the onset for yield can be described by a dislocation mechanism. The total energy absorbed during impact, G Ic, is a complex function of the morphology with no single identifiable morphological characteristic that dominates its value. The heterogeneous, dispersed morphology of the PP block copolymer initiates an additional energy absorption mode compared to the homo-polymer and the random-copolymer.

From Granular Flux Model to Traffic Flow Description

A description of highway traffic flow is proposed, based on a flux model originally developed for granular gases in a compartmentalized setup. Results from a pilot study of the highway A58 in the Netherlands are presented, followed by a discussion of possible improvements and applications of the model.