Protein-protein interactions are essential for various biological processes including cell metabolism, muscle contraction, and signal transduction. The dissertation describes a study of the... Show moreProtein-protein interactions are essential for various biological processes including cell metabolism, muscle contraction, and signal transduction. The dissertation describes a study of the interaction between the proteins cytochrome c and cytochrome c peroxidase by electron paramagnetic resonance spectroscopy (EPR). A spin label containing an unpaired electron was placed at the surface of one of the proteins. The combination of spin labelling and EPR provided novel information on the structure and dynamics of the proteins. Show less
Geometric frustration occurs when local order cannot propagate through space. A common example is the surface of a soccer ball, which cannot be tiled with hexaganons only. Geometric frustration can... Show moreGeometric frustration occurs when local order cannot propagate through space. A common example is the surface of a soccer ball, which cannot be tiled with hexaganons only. Geometric frustration can also be present in materials. In fact, geometry can act as an instrument to design the mechanical, optical or physical properties of fluids and solids. The first two parts of this thesis discuss frustrated liquid crystals confined to droplets of various shapes and sizes. The droplet shape determines the orientation of the liquid crystal molecules and in turn its response to light. In the final part we study the fracture mechanics of curved elastic plates. By tuning the curvature of the plate, the critical length at which the crack starts growing can be controlled. Finally, we find that the path that the crack takes depends on the curvature. Show less
Life’s building block is a cell. Different cell types are differentiated by specific functional properties. A white blood cell, for instance, can get rid of bacteria and many muscle cells contract... Show moreLife’s building block is a cell. Different cell types are differentiated by specific functional properties. A white blood cell, for instance, can get rid of bacteria and many muscle cells contract together for proper muscle function. Deformation and force exertion play important roles in these processes. Bacteria have to be physically engulfed by the white blood cell, and the muscle cell has to contract in the right way. In this research we measured how much force cells exert and simultaneously visualized specific proteins. A newly developed technique enabled the visualization of the nanometer-structure of cellular adhesions. We also examined the relationship between cellular shape and orientation of an intracellular network of protein (actin). We discovered that the signal of yet another protein (p130Cas) alters the mechanical behavior of the cell when the stiffness outside the cell changes. Finally, we also examined the structure of other proteins (tubulin and H2B) during cell division. In all these processes we measured how much force a cell exerts on its environment. The results provide important insights in the mechanical component of cellular function and their role in life Show less
This work discusses the flow of granular materials (e.g. sand). Even though a single particle is a simple object, the collective behavior of billions of particles can be very complex. In a... Show moreThis work discusses the flow of granular materials (e.g. sand). Even though a single particle is a simple object, the collective behavior of billions of particles can be very complex. In a surprisingly large amount of cases, it is not exactly known how a granular material behaves, and this while these kinds of materials are omnipresent in everyday life, industry, and nature Similar to materials such as water, which can occur as ice, liquid water and vapor, sand can also exist in different phases of matter. If you for instance walk on the beach, sand behaves like a solid, but if you pour it out of your shoes afterwards, it flows like a liquid. This thesis is dedicated to experiments where we investigate what happens when you try to "liquefy" sand by weakly vibrating it. We use an experimental setup which enables us to study how much stress is required to make sand flow, depending on the desired flow rate and the amount of vibrations. These experiments reveal several physical principles that turn out to be important towards understanding the unique behavior of these kinds of materials Show less
In this thesis, we perform fundamental spectroscopic studies of organic fluorescent chromophores (dyes) for temperatures from 1-10 Kelvins. We use confocal and wide-field fluorescence microscopy... Show moreIn this thesis, we perform fundamental spectroscopic studies of organic fluorescent chromophores (dyes) for temperatures from 1-10 Kelvins. We use confocal and wide-field fluorescence microscopy techniques. We analyzed the spectroscopic properties of well-known dyes, such as perylene, terrylene and dibenzoterrylene embedded in different solids. By doing this systematically, we correlate the presence of methyl groups in the host-crystal (Chapter 2) with the strong spectral diffusion of the resonant line, which lead to spectral broadening and frequency instabilities of the guest DBT. In contrast, the absence or substitution of methyl groups by chloride showed any of these effects. We found that the fluorescent properties of perylene in ortho-dichlorobenzene and terrylene in para-dichlorobenzene solids were those expected for the single quantum system. No reports of lifetime-limited excitation lines for perylene were ever reported till now. Even further, we report the coupling of an acoustic wave created by a macroscopic object to the optical response of a single molecule. The shot-noise limited sensitivity from the response of the single molecule to the displacement of the fork's prongs was found to be in the sub-nanometer regime. Perylene in o-DCB will be used to perform the single molecule all-optical transistor proposed in the introduction. Show less
Since the discovery of the right-handed helical structure of DNA, 61 years have passed. The DNA molecule, which encodes genetic information, is also found twisted into coils. This extra twist of... Show moreSince the discovery of the right-handed helical structure of DNA, 61 years have passed. The DNA molecule, which encodes genetic information, is also found twisted into coils. This extra twist of the helical structure, called supercoiling, plays important roles in both DNA compaction and gene regulation. The DNA in eukaryotic cells is packaged into chromatin. Using single-molecule force spectroscopy, I resolved force/torque induced structural changes of DNA and chromatin fibers. I showed that the structural changes of chromatin fibers can be described by four conformations. I showed for the first time the folding and unfolding of a chromatin fiber under torsion. Th e anisotropic response of chromatin fibers to supercoiling reflects its leftŸ-handed chirality. These findings give a detailed structural insight of a supercoiled chromatin fiber, yielding a better understanding of the response of chromatin during transcription Show less
Cosmological inflation is the most successful theory that explains the homogeneity and flatness of the early universe. It also provides a quantum origin for the primordial perturbations that we... Show moreCosmological inflation is the most successful theory that explains the homogeneity and flatness of the early universe. It also provides a quantum origin for the primordial perturbations that we observe in the Cosmic Microwave Background Radiation (CMB). The simplest models make use of a single scalar field, which produces the exponential expansion of the early universe. In this thesis the effects of additional heavy fields is studied from several points of view. On the one hand, possible signatures due to the presence of such fields are searched for in the current CMB data, and a new formalism is developed, allowing the analytical study of features arising from the presence of these fields. On the other hand, in the context of supergravity theories, the presence of additional heavy fields is ubiquitous, and their stability is needed in order to not spoil the basic predictions of single-field inflation. The viability of inflation in the presence of an additional heavy supersymmetric sector is studied, and several stability constraints are derived. The conclusion is that CMB data might be already sensitive enough to the presence of additional heavy fields, and that the landscape of supergravity scenarios which stabilize these fields is very constrained Show less
This thesis is devoted to applications of string theoretic methods of holography to strongly coupled phases of quantum field theories. In chapter 2 we consider a finite-density system of quarks,... Show moreThis thesis is devoted to applications of string theoretic methods of holography to strongly coupled phases of quantum field theories. In chapter 2 we consider a finite-density system of quarks, realized holographically by a probe brane in Anti-de Sitter space, with a non-trivial gauge field background on its world-volume. We reproduce the holographic zero-sound in the longitudinal channel of the current-current correlation function. We generalize this result to the case of a non-vanishing background magnetic field. This field leads to a gap in the zero-sound mode, which scales proportionally to the magnitude of the field when it is small. In chapter 3 we study the classical dynamics of the tachyon field in an AdS background described by the tachyon-Dirac-Born-Infeld action. By considering a black hole in AdS space and switching on a non-vanishing background gauge field we obtained a holographic model of conformal symmetry breaking in a strongly coupled system at finite temperature and charge density. In chapter 4 we provide the exact string theoretic description of a quantum field theory at finite temperature and charge density Show less
A time integrated search for cosmic neutrinos is discussed in this thesis using four years of data collected by the ANTARES experiment. No statistically significant signal was found, therefore... Show moreA time integrated search for cosmic neutrinos is discussed in this thesis using four years of data collected by the ANTARES experiment. No statistically significant signal was found, therefore upper limits on the neutrino flux were derived. Limits for specific models of RX J1713.7-3946, Vela X and Crab Nebula which include information on the source morphology and spectrum, are also given. Show less
In this PhD thesis we investigated the feasibility to build a spin-flip laser, a spintronic device recently proposed by Kadigrobov that can emit radiation within the so-called terahertz gap. In the... Show moreIn this PhD thesis we investigated the feasibility to build a spin-flip laser, a spintronic device recently proposed by Kadigrobov that can emit radiation within the so-called terahertz gap. In the first part of this thesis, we describe point contact spectroscopy experiments which were meant as a first step to the final observation of radiation coming from the spin-flip laser. The ferromagnet used in our spin-flip laser is SmCo5, a hard ferromagnet with a very large coercive field. The next chapter of the thesis describes the sputter growth of thin films of Sm-Co. We found that one of the main problems of building a spin-flip laser is the issue of spin current injection and the relaxation of the spin current in an adjacent layer. In the last chapters of the thesis, we use the ferromagnetic resonance technique to study how the spin currents behave at the interface of a ferromagnet and a normal metal with large spin-orbit coupling, such as Pt or Pd or a normal metal with weak spin-orbit coupling, such as Cu. Show less
