On the Transition from Bulk to Ordered Form of Water: A Theoretical Model to Calculate Adhesion Force Due to Capillary and van der Waals Interaction

The adhesion force due to capillary interaction between two hydrophilic surfaces is strongly dependent on the partial pressure of water and is often calculated using the Kelvin equation. The validity of the Kelvin equation is questionable at low relative humidity (RH) of water, like in high vacuum and dry nitrogen environments, where water is only present as layers of several molecules thick at the surfaces. A model from ordered to bulk form of water has been developed using the Brunauer, Emmett, and Teller adsorption model. The results show that the adhesion force calculated using the Young–Laplace and Kelvin equations at low (5–30 %) RH is underestimated. The total adhesion force shows changes when the RH is changed from 0 to 100 %. In dry conditions, at RH below 10 %, the total adhesion force is contributed by the van der Waals interaction due to solid–solid contact. The total adhesion force then increases and remains constant being equal to the superposition of van der Waals interaction due to solid–solid contact and van der Waals interaction due to adsorbed water layers on the surfaces. The total adhesion force further increases slowly with the increase in RH incorporating capillary forces and then decreases at very high RH due to screening of van der Waals forces. This change in adhesion force occurs from solid–solid interaction to ordered form of water at low RH and from ordered form to bulk form of water at high RH along with the screening effect of van der Waals interaction. The results have been compared with the experiments and it has been seen that at small length scales, the model is in agreement with the existing experimental data. However, at large length scales roughness of the surfaces should be taken into account

Early mobilisation versus plaster immobilisation of simple elbow dislocations: Results of the FuncSiE multicentre randomised clinical trial

Background/aim To compare outcome of early mobilisation and plaster immobilisation in patients with a simple elbow dislocation. We hypothesised that early mobilisation would result in earlier functional recovery. Methods From August 2009 to September 2012, 100 adult patients with a simple elbow dislocation were enrolled in this multicentre randomised controlled trial. Patients were randomised to early mobilisation (n=48) or 3 weeks plaster immobilisation (n=52). Primary outcome measure was the Quick Disabilities of the Arm, Shoulder, and Hand (Quick-DASH) score. Secondary outcomes were the Oxford Elbow Score, Mayo Elbow Performance Index, pain, range of motion, complications and activity resumption. Patients were followed for 1 year. Results Quick-DASH scores at 1 year were 4.0 (95% CI 0.9 to 7.1) points in the early mobilisation group versus 4.2 (95% CI 1.2 to 7.2) in the plaster immobilisation group. At 6 weeks, early mobilised patients reported less disability (Quick-DASH 12 (95% CI 9 to 15) points vs 19 (95% CI 16 to 22); p<0.05) and had a larger arc of flexion and extension (121° (95% CI 115° to 127°) vs 102° (95% CI 96° to 108°); p<0.05). Patients returned to work sooner after early mobilisation (10 vs 18 days; p=0.020). Complications occurred in 12 patients; this was unrelated to treatment. No recurrent dislocations occurred. Conclusions Early active mobilisation is a safe and effective treatment for simple elbow dislocations. Patients recovered faster and returned to work earlier without increasing the complication rate. No evidence was found supporting treatment benefit at 1 year

On the generation of surfable ship waves in a circular pool: Part I: physical background & wave pool design

Surfing as a sport is growing. Locations with good surfing conditions are limited and therefore crowded. The growing interest for the sport led to a new concept, surf pools. Surf pools mimic good surfing conditions found along the coast. This thesis is about the technical feasibility of a new surf pool concept, creating surfing waves in a circular pool by towing ship-like hulls around. The hulls are towed along the outer wall of the pool and waves generated by these hulls propagate inwards and break on an island in the middle of the pool. The waves break around the island which theoretically leads to endless surf rides. The initial design as suggested by Webber is based on a pool diameter of 200 m and a water depth of 3 m. The objective of his design to generate 2 m high waves for intermediate to expert surfers. Because of the lack of knowledge about ship induced waves in such an enclosed environment, a physical and a numerical model study are carried out. Various hull sizes are towed in a towing tank and the surface elevations are measured and analyzed. From this physical model study it was concluded that the wave pattern behind a hull is the result of a complex interference pattern and is easily disturbed when the velocity beside the hull becomes large. The difficulties of wave generation by a hull therefore lie in the return currents. The magnitude of the return flow is a function of the blockage, the percentage of the cross section of the pool blocked by the hull. In case the blocking and the velocity are too large, water accumulates in front of the hull and the wave field behind the hull is disturbed, the so-called trans-critical regime. Maximum wave height is found when to towing a hull at the onset of this trans-critical regime. The initial wave pool design is evaluated, and it was concluded that the water depth of 3 m is insufficient to generate the desired 2 m waves. Therefore an amended design is presented. Water depth is increased to 7 m and the generation area is enlarged to reduce the blockage to 5%. The large generation area causes a weak return flow and therefore less turbulence in the pool. In such a cross section it is considered technically possible to generate 2 m high surfable waves.Civil Engineering and Geoscience

Alongshore variability of nourished and natural beaches

Alongshore variability in topography (i.e. height differences in bed level along the coast) can exist on both natural and nourished beaches. An important question prior to implementation of a nourishment project is how alongshore variability is going to evolve and, related to this variability, the effect of the project upon swimmer safety. In this thesis observations and analysis of the alongshore variability in nearshore topography of a nourished Dutch beach are discussed. Three and a half years of nearly monthly measurements with a jetski survey system and GPS backpack show in detail how the topography of the beach changed after installation of a large nourishment. The results show that the first period just after implementation of the nourishment showed similar magnitudes of variability than the total period investigated, and no apparent effect of the nourishment shape on the variability could be discerned. Moreover, changes in topographic variability were found to be much slower than previous observations, f.i. at the US east coast or Australian east coast. The formation process of alongshore variability is examined in more detail by examining field measurements of an Australian natural beach and using conceptual numerical modelling. Special attention herein is paid to the effect that different sea states (swell vs. wind sea) have upon flow circulation and the formation of topographic variability in the nearshore, to put the results in the thesis into the context of the Dutch wind sea wave climate. This analysis has shown that the typical Dutch wind sea wave climate is likely to result in slower formation of alongshore variability due to the short wave period (the time between consecutive waves) and high wave angles (angle at which it waves approach shore) in this wave field. Very low frequency flow velocity oscillations can be forced by variations in wave height and are related to transient rip currents. Analyses in the thesis show that these oscillations are typically small in the Dutch wave climate. However, the steep profile of a nourished beach could enhance the magnitude of these motions, such that substantial fluctuations in current can be present at a Dutch nourished beach.Hydraulic EngineeringCivil Engineering and Geoscience

Characterizing three-dimensional wave-driven morphological diffusivity in the nearshore

Accurate predictions of the fate of sand nourishments in the nearshore are critical to the management of eroding shorelines. The effectiveness of these nourishments depends on wave and current conditions, which vary with nourishment shape, size, and location. Here, observations of artificial sand mounds, including a new laboratory experiment, are examined to characterize the impact of a range of wave and current conditions on mound evolution and to inform coastal management techniques.Coastal Engineerin

Relating friction and adhesion for single smooth contact by means of models and experiments

The relation between adhesion and the friction force measurements for a single smooth contact has been studied by means of sliding and pull-off experiments in ambient as well as under high vacuum (HV) conditions. The experimental results have been analysed in several ways using contact models. First, it is shown how the appropriate adhesion regime can be determined based on the analysis of Maugis–Dugdale (M–D). Further, a modified M–D model incorporating meniscus forces are discussed. This model is able to calculate the contact radius for cases where meniscus forces due to the presence of water are dominating. The approach will be illustrated by means of adhesion and friction force measurements between a silicon ball and a glass flat surface (Si–glass interface) performed on a novel designed vacuum based adhesion and friction tester. Under HV conditions, meniscus forces are not dominating and only van der Waals interactions are present. Combining the models with the measured adhesion and friction force data, the work of adhesion and the shear stress present between the Si–glass interface can be determined. According to the theory, the extent of the adhesive zone increases with an increase in the relative humidity (RH). When comparing HV conditions with ambient conditions, it has been found that the adhesion as well as friction force is significantly lower under high vacuum conditions for the Si–glass interface. When comparing modelling results with experiments, it can be concluded that the trends from theoretical predictions are in good agreement with the measurements, both for the HV and ambient regime