A foam is not a homogeneous material, but consists of a large number of small air bubbles. The whole is more than the sum of its parts: even though the behavior of a single bubble is easy... Show more A foam is not a homogeneous material, but consists of a large number of small air bubbles. The whole is more than the sum of its parts: even though the behavior of a single bubble is easy to understand, their collective behavior is much more complicated and completely different. The reason for this is that a deformation of the whole is only reflected partially in the deformation of individual bubbles. Instead, a large part of the deformation is reflected in a change in the structure of the foam. In this thesis, we investigate this change in structure. In the first part of this thesis, we use a simple microscopic computer model to simulate the response of a foam when it is deformed by a tiny amount, and measure how far we need to deform the foam until it is irreversibly deformed. In the second part of this thesis, we focus on an experiment, where we measure the response of a two-dimensional foam when it is deformed at the edges. We observe that the response of bubbles in the center of the foam qualitatively depends on how densely the bubbles are packed together. Show less
Jolles, D.D.; Supekar, K.; Richardson, J.; Tenison, C.; Ashkenazi, S.; Rosenberg-Lee, M.; ... ; Menon, V. 2016
In this thesis we first studied the start of the epidemiologic transition in rural Ghana and describe the changes in mortality. This is followed by studies on the compression of mortality and... Show moreIn this thesis we first studied the start of the epidemiologic transition in rural Ghana and describe the changes in mortality. This is followed by studies on the compression of mortality and morbidity during the transition in Japan and the Netherlands. Finally, we examined the plasticity of mortality at old age. Studying old-age mortality during the epidemiologic transition is of great importance now that an increasing number of people reach old-age. The main findings from this thesis are that overall, when countries shift from a pre- to a post-transitional stage, mortality decreases and an increasing number of people live up to old age. In addition, mortality is compressed to a narrower age interval in which most of the annual deaths occur. This compression of mortality reaches a limit as soon as countries enter the post-transitional era. Furthermore, during the last stage of the epidemiologic transition we have observed that life expectancy without disability is increasing parallel to the increase in life expectancy, indicating a compression of disability. Finally we found that, mortality remains highly plastic up to old age, as indicated by the continuous rise in life expectancy and the shift of the age-at-death distribution towards higher ages. Show less
Many materials, like foams, emulsions, suspensions and granular media obtain finite rigidity once their constituent particles are brought in contact. Nevertheless, all these materials can be made... Show moreMany materials, like foams, emulsions, suspensions and granular media obtain finite rigidity once their constituent particles are brought in contact. Nevertheless, all these materials can be made to flow by the application of relatively small stresses. By varying thermodynamic (temperature or density) and mechanical (applied stress) variables, one can bring about a transition from a freely flowing to a jammed state. What is the elastic response of foams close to the jamming point? How much can these materials be loaded before they flow? What is their behavior like in the bulk? These problems are of great interest in academics, as well as industrial applications (oil/gas extraction, cosmetics, pharmaceuticals and food processes). I study the transition from the flowing to the non-flowing regime in foams and analyze the non-affine behavior at this critical point. Additionally, whereas the usual rheological approach is to study the shear, I have developed a technique to measure compressive response in a real-world, foam system, taking gravity and temperature fluctuations into account. Show less
It is well known that complex mental abilities develop at least until late adolescence. Yet, there are also skills that children master perfectly, sometimes even better than adults. The goal of... Show moreIt is well known that complex mental abilities develop at least until late adolescence. Yet, there are also skills that children master perfectly, sometimes even better than adults. The goal of this thesis was to learn more about the possibilities of cognitive functioning in children and young adults, and the constraints set by the developing brain. An fMRI training approach was used to examine age- and experience-related effects in the development of working memory and resting-state functional connectivity. More specifically, we studied age differences on task performance and brain activation during a working memory task with various demands and difficulty levels, both before and after 6 weeks of practice with the task. In addition, to learn more about the interaction between different brain regions, we also examined age differences and practice effects on functional connectivity during resting-state. Show less
