Control of Dynamic Hopf Bifurcations

The slow passage through a Hopf bifurcation leads to the delayed appearance of large amplitude oscillations. We construct a smooth scalar feedback control which suppresses the delay and causes the system to follow a stable equilibrium branch. This feature can be used to detect in time the loss of stability of an ageing device. As a by-product, we obtain results on the slow passage through a bifurcation with double zero eigenvalue, described by a singularly perturbed cubic Lienard equation.Comment: 25 pages, 4 figure

Organic production of strawberries

Organic fruit and vegetable growing is increasing world-wide, but knowledge concerning best practice is generally empirical rather than field-based. This thesis extends knowledge concerning best practice in Swedish organic strawberry (Fragaria x ananassa (Duch)) production, which is usually based on a two-year crop. A series of four field experiments were conducted at Rånna Experimental Station (58°27’N, 13°51’E) on the strawberry cultivars ‘Honeoye’, ‘Cavendish’ and ‘Korona’. Specific objectives were to investigate: the effects of different organic fertilisers combined with degradable plastic mulch on plant establishment, yield and nitrogen mineralisation in soil; the efficiency of biological pest control using a predatory mite against strawberry mites (Phytonemus pallidus); innate within-field dispersal capacity of strawberry mite; the effects of pyrethrum combined with fleece covering on damage by strawberry blossom weevil (Anthonomus rubi). Plant establishment measured as fruit yield was improved by 60% when degradable plastic mulch was used, probably through more flowers being initiated. Additional applications of organic fertiliser during the cropping period had little effect on yield. Biological control using the predatory mite Neoseiulus cucumeris reduced strawberry mite populations by up to 50% but there was no increase in yield. It was difficult to draw clear conclusions from the strawberry mite dispersal pilot study, but mites tended to move more easily along planted rows than between rows. Pyrethrum alone had no visible effect on flower bud damage by strawberry blossom weevil but when combined with fleece covering it reduced the number of ‘Honeoye’ buds damaged by weevils by approximately 10%. This was followed by a corresponding increase in cultivar yield. Similar effects were not recorded in the other cultivar examined. The study identified the following practical consequences for organic strawberry production: • Choice of pest-resistant cultivars is extremely important for success. • Use of clean plants and an efficient predatory species is essential in controlling mite infestations. • Use of degradable plastic mulching is recommended. • Fleece covering is recommended for protecting some cultivars against strawberry blossom weevils. • Only an initial starter fertilisation is required. • Use of pyrethrum is not recommended

Hysteresis in Adiabatic Dynamical Systems: an Introduction

We give a nontechnical description of the behaviour of dynamical systems governed by two distinct time scales. We discuss in particular memory effects, such as bifurcation delay and hysteresis, and comment the scaling behaviour of hysteresis cycles. These properties are illustrated on a few simple examples.Comment: 28 pages, 10 ps figures, AMS-LaTeX. This is the introduction of my Ph.D. dissertation, available at http://dpwww.epfl.ch/instituts/ipt/berglund/these.htm

Intramolecular fluorescence correlation spectroscopy in a feedback tracking microscope

We derive the statistics of the signals generated by shape fluctuations of large molecules studied by feedback tracking microscopy. We account for the influence of intramolecular dynamics on the response of the tracking system, and derive a general expression for the fluorescence autocorrelation function that applies when those dynamics are linear. We show that tracking provides enhanced sensitivity to translational diffusion, molecular size, heterogeneity and long time-scale decays in comparison to traditional fluorescence correlation spectroscopy. We demonstrate our approach by using a three-dimensional tracking microscope to study genomic $\lambda$-phage DNA molecules with various fluorescence label configurations.Comment: 11 pages, 5 figures, supplemental info: http://minty.stanford.edu/papers/Publications/McHale10aSI.pd

Per Berglund Professor of Physics travels to Germany

The CIE grant provided financial support to travel in May 2013 to Hamburg, Germany to visit DESY, the German Electron Synchrotron, one of the world\u27s leading particle accelerators used to study the fundamental constituents of nature and their interactions. My local expenses were covered by a Collaborative Research Center (SFB) fellowship that I was awarded through SFB’s 676 Particle, Strings and the Early Universe--The Structure of Matter and Space-Time hosted by DESY and the University of Hamburg. Together these awards facilitated two months of visits and collaborations with theoretical physicists at DESY in 2013

A framework to study learning in a complex learning environment

This paper presents a framework for analysing variations in the ways in which students experience learning in complex, computer-supported environments. It involves an application of phenomenography extended to encompass the content of the learning as well as its context. Concepts from activity theory are drawn upon to enlarge the scope of phenomenography. By applying this framework, researchers, as well as course organisers, may gain insights into how courses are experienced by the participants and can thus develop these in a way that will be perceived as improvement by the course participants. Within this predominantly phenomenographic approach, the focus is upon the content of the learning experience, giving the ability to relate learning outcomes to the experience of the learning environment. Hence, insights can be gained that can help to improve learning environments and thereby outcomes. A case study illustrating the kinds of results than can be obtained when using the framework is provided

Nonexponential statistics of fluorescence photobleaching

In this paper, I consider theoretical models of the decay via photobleaching of a sample of surface-immobilized fluorescent molecules excited by a spatially varying laser intensity profile. I show that, with mild restrictions on the photobleaching mechanism, the fluorescence decay measured in a nonuniform excitation profile is always nonexponential. Under the same conditions, the fluorescence decay can always be approximated by a discrete sum of exponentials. A particular example is given in which a homogeneous population of fluorophores with a single (intensity-dependent) photobleaching lifetime, when illuminated by a Gaussian laser, exhibits power law fluorescence decay at long times. These results indicate that the observation of multiple exponentials in single molecule or ensemble photobleaching lifetime measurements can arise solely as an artifact of a spatially varying laser profile and is not necessarily indicative of heterogeneity in molecular internal states, conformation, or local environment