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Physics With Industry
The second workshop Physics with Industry was organized in 2011 by the Foundation FOM and Technology Foundation STW at the Lorentz Center at Leiden, the Netherlands.
Sixty-four scientists participated in the workshop 2011, ranging from PhD students to professors. These physicists (and researchers from related disciplines) spent a week working in groups on five industrial problems, which were selected by a programme committee from proposals put forward by industry. Following an introduction to the various problems by the companies on Monday, the participants worked on the problems in groups for the rest of the week. On the last day, the groups presented their findings to the companies. A novelty in the 2011 workshop was that some groups performed real experiments at the laboratories of Leiden University.
Besides the scientific outcomes, the workshop also resulted in new public private contacts that may lead to future collaborations. Participants were mostly driven by the shear pleasure of applying their physics knowledge to new problems, the desire to enrich their scientific network and the interest in gaining hands-on experience with industrial R&D processes. Companies benefited from the scientific input they received and participating in the workshop enlarged their academic network.
The five industrial problems discussed during the week were collected via an open call for proposals in spring 2011. A programme committee selected the five 'best problems' for the workshop. The selection criteria used by the committee were:
The committee selected problems from the companies Avery Dennison, FEI Company, Nano4Imaging, OcÚ and Unilever. Four large companies and one SME.
The proceedings are available via the FOM website. Below is a summary of the five cases.
Avery Dennison: Develop self adhesive to stick on moist and icy substrates
Normal acrylic-based adhesives that stick to dry surfaces, do not stick to surfaces with a water film. The water decreases the Hamaker constant, which indicates the strength of the Van der Waals forces, by a factor 10. The time needed to squeeze out the water by applying pressure to a label on top of a wet surface, is too long for normal applications. Approaches to remove, use and penetrate the moisture layer are proposed. This work focuses on proposals for water removal and this case is analyzed theoretically and tested experimentally. Pores are needed to transport the water away from the gap between the substrate and the adhesive layer. We show experimentally that adhesives with pores (50 μm diameter, 1 mm spacing) have a larger pull-off force on wet surfaces after applying pressure than adhesives without pores. Theoretical calculations for a 20μm thick adhesive layer of 645 mm2 surface area with 800 holes of 10μm diameter, show that the maximum volume of water retainable in the capillaries is 1.5 .10 -12 m3. This value is 500 times less than the volume of water squeezed out when the layer is reduced to 1μm. Therefore pores need to be made through both the adhesive and film layer where the water can evaporate or an absorbance layer is needed. Alternative strategies proposed to improve adhesion performance on moist icy surfaces include addition of polysaccharides, (poly)electrolytes, nanofibres, functionalized superhydrophobic and superhydrophilic patterns of the adhesive layer.
FEI Electron Optics BV: Field of inserted charges during Scanning Electron Microscopy of non-conducting samples
Three different approaches to calculating the electric potential in an inhomogeneous dielectric next to vacuum due to a charge distribution built up by the electron beam are investigated. An analytical solution for the electric potential cannot be found by means of the image charge method or Fourier analysis, both of which do work for a homogenous dielectric with a planar interface to vacuum. A Born approximation gives a good approach to the real electric potential in a homogenous dielectric up to a relative dielectric constant of 5. With this knowledge the electric potential of an inhomogenous dielectric is calculated. Also the electric field is calculated by means of a particle-mesh method. Some inhomogeneous dielectric configurations are calculated and their bound charges are studied. Such a method can yield accurate calculations of the electric potential and can give quantitative insight in the charging process.
A capacitor model is described to estimate the potential due to the charge build up. It describes the potential build up in the first microseconds of the charging. Thereafter, it seems that more processes have to be taken into account to describe the potential well. This potential can further be used in a macroscopic approach to the collective motion of the electrons described by the Boltzmann transport equations or a derived density model, which can be a feasible alternative approximation to the more commonly used Monte-Carlo simulation of individual trajectories.
Nano4Imaging BV: MRI imaging of instruments - design of markers in relation to artefact size
Nano4Imaging provides magnetic markers that are fit to be put on guidewires used in MRI scanners. The size of the artefact that is induced by these markers in a MRI image depends on the geometrical and magnetic properties of the marker and the imaging parameters set on the MRI scanner such as echo time and voxel size. During the one-week Physics with Industry 2011 workshop we have developed a Mathematica model that predicts the size of the artefact as a function of these settings.
We have demonstrated that using elementary physics, we can model an MR image for arbitrary marker properties and MRI settings. We have captured this model into a numerical simulation which produces realistic images for typical marker and MRI settings. The important MRI and marker properties are input parameters for the simulation, such that a wide variety of scenarios can be investigated.
The dependency of the image appearance on the individual parameters can be understood physically. With the proposed model the artefacts generated by (super)paramagnetic markers can be simulated, without the need to use an MRI scanner. This will contribute to the rapid customization of ideally sized and shaped artefacts at low cost and rapid prototyping.
Unilever R&D: Structuring with anisotropic colloids
Structure is an important factor in food. One of the ways to provide structure to foods is by using bubbles and foams. However, they need to be stabilized. One way of doing this is by covering them with microscopic rods. These rods self-assemble at the surface, yielding a stable bubble. The goal of this work is to gain a better understanding into how this self-assembly works using analytical calculations, experiments and simulations.
We have presented the results of our different approaches to the problem of self-assembly on curved surfaces. What we found, firstly, is that although some elements are present in the literature, a lot more research is needed.
We have taken the first steps by looking at the analytical behaviour of the defects and the general geometry of such surfaces. In order to make the analytical calculation more realistic, one might want to add a directional energy cost for domains that fit together but have a different orientation of the rods. Furthermore, a better understanding of the exact energy cost associated with different boundaries is needed.
Our experiments show that the key features of the microscopic self-assembly, such as the formation of domains, can be captured by macroscopic systems. This suggests that the main effects are geometric in nature. Our experiments we limited by the behaviour of the rods at the surface. In the microscopic case, the rods can deform the surface of the bubble and they hardly overlap. We could not reproduce this in our macroscopic systems. As a next step it would be useful to repeat the experiments with a material that can be deformed by the rods and where the rods have more buoyancy.
Finally, the simulations we performed were quite successful in reproducing the general behaviour of the self-assembly, but it seems that the domains that are formed are somewhat larger than in reality. To make these simulations more realistic, one would have to change the aspect ratio of the rods, as well as the size of the bubble and the entire system (easily within reach with these methods). It would be useful to further pursue the question of polydispersity in the rod population along, as demonstrated here for DPD simulations.
Although more realistic simulations would be computationally more expensive, it is still possible to do them in a reasonable amount of time within our current framework. Thus, even though this is only preliminary research, we see some promising results.
OcÚ: Sticky Bubbles
We discuss the physical forces that are required to remove an air bubble immersed in a liquid from a corner. This is relevant for inkjet printing technology, as the presence of air bubbles in the channels of a printhead perturbs the jetting of droplets. A simple strategy to remove the bubble is to flush the ink past the bubble by providing a high pressure pulse. In this report we first compute the viscous drag forces that such a flow exerts on the bubble. Then, we compare this to the \sticking forces" on the bubble, due to the capillary interaction with the wall. From this we can estimate the required flow velocities for bubble removal, as a function of channel geometry, contact angle and ink properties. Finally, we investigate other ways to exert a force on a trapped bubble. In particular we focus on forces induced by electric fields which can alter the contact angle of the drop, or by locally applying thermal gradients. Once again, these forces are compared to the sticking forces to identify the parameters where the bubble can be removed.
The removal of a bubble from a corner has been investigated. When the bubble is not attached to the wall, but it is near the corner, the normal flushing operation should be sufficient to remove the bubble. The flow pattern that results from the flushing was calculated. The velocity close to the corner is still significant. The drag force on the bubble has been estimated.
In order to determine whether the viscous drag is sufficient to remove the bubble from the corner, the retaining force has been calculated by an analysis of the surface energy of a bubble in the corner and of a bubble in contact with a at wall. For any finite contact angle, the energy of a bubble is lower in the corner, which gives rise to a retaining force. This retaining force is large enough to withstand the drag that would arise if the liquid were ejected with sonic (in air) speed from the nozzle, for a contact angle of 90. For lower contact angles, the retaining force diminishes quickly. For a perfectly wetting surface, the capillary force on the bubble is zero.
For contact angles of 90 or more, the bubble cannot be removed by flushing. Preventing the bubble from reaching the corner becomes very important in this case. Two effects that may be used to push the bubble away from the corner were examined. The first effect is thermal migration. Three kinds of thermal influence have been considered in this report. One of them is the Marangoni effect that moves the bubble due to the dependence of the surface tension on temperature. This is the dominant contribution to the thermal migration of the bubble. The other two thermal effects are thermal convection and transfer of the vapor across the bubble due to the temperature difference at the opposite sides of the bubble. These effects are both negligible. Of these three thermal effects, only the Marangoni effect is significant. The second effect is electromigration, where an electric field induces a force on the bubble. This force is too small to overcome secondary Bjerknes force, though electrowetting may be sufficient to reduce the retaining force enough to enable flushing.
Since bubbles can only be removed when the contact angle is small (hydrophyllic), controlling the wetting properties in the channel is crucial. Thermal migration may be used to prevent the bubble from reaching the corner, so that less ink is needed to flush the bubble. To use this effect, the corner must be kept cool. If these efforts prove to be insufficient, electrowetting can be investigated to enable flushing the bubble.