Critical point (thermodynamics)

In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. One example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions under which a liquid and its vapor can coexist. At higher temperatures, the gas cannot be liquefied by pressure alone. At the critical point, defined by a critical temperature Tc and a critical pressure pc, phase boundaries vanish. Other examples include the liquid–liquid critical points in mixtures, and the ferromagnet–paramagnet transition (Curie temperature) in the absence of an external magnetic field. Liquid–vapor critical point Overview For simplicity and clarity, the generic notion of critical point is best introduced by discussing a specific example, the vapor–liquid critical point. This was the first critical point to be discovered, and it is still the best known and most studied one. The figure to the right shows the schemat

GHz spectroscopy on a rare-earth quantum magnet

Lev Bergsma

In this report, spectroscopy was used to track the energy of different states in a LiHoF4 sample around its critical temperature. The experimental setup consist of a Cooper cavity with the sample, an antenna and a resonator inside. This cavity is attached to a probe and put inside a cooled well and a transverse magnetic field. Two different frequency ranges were examined around, f0 = (1.18732 0.00001) Ghz and f0 = (3.32940 0.00001) Ghz. The energy difference between states was successfully tracked for different temperatures thanks to the Q factor plots. However, the lack of sufficient data points and the unexpected behaviour around Tc (there are no gaps and no discontinuities), implies that this is only the beginning of this experiment and more time and measurements is needed to claim reasonable results.

2022

Second harmonic scattering of water in a biological context

Tereza Schönfeldová

Water is one of the most abundant molecules in the universe. It forms a hydrogen bond network with unique structure and dynamics. Hydrogen bonding is highly relevant to many biological processes including membrane formation, protein folding, adsorption, and lubrication. However, the underlying molecular interactions within water and between water and membranes are difficult to study and hence not fully understood. In this thesis, we utilize second harmonic scattering (SHS) to investigate inhomogeneities in bulk water, probe structural changes in lipid bilayer membranes, and detect and characterize protein-membrane interactions. Firstly, we explore the structure of bulk water in comparison to a common model liquid, carbon tetrachloride. Combining SHS measurements and molecular dynamics simulations, we report on the nanoscale inhomogeneities in bulk water that occur on femtosecond timescales. We attribute the rise in the coherent second harmonic (SH) intensity to charge density fluctuations with enhanced nanoscale polarizabilities around transient voids. Even though the transient voids also exist in carbon tetrachloride, they do not show polarizability fluctuations and the SH response of carbon tetrachloride thus corresponds to that of a model isotropic liquid. Then, we study the structure of water around large unilamellar vesicles (LUVs), which serve as a model for cell membranes. We focus on the role of water during the lipid melting phase transition. Using SHS, differential scanning calorimetry, and temperature-dependent zeta-potential measurements, we investigate the structural changes of hydration shells around the vesicle membrane. We use lipids with phosphoserine, phosphocholine, and phosphate headgroups and observe different hydration behavior upon the lipid melting transition depending on the membrane composition. Next, we characterize the interface of membranes composed of lipids containing phosphocholine and phosphate headgroups in solutions with different CaCl2 concentrations. We observe that the SH intensity is decreasing with the addition of CaCl2, which we attribute to the formation of a condensed layer of Na+ counterions on the inner leaflet of the LUVs. Additionally, we extract the vesicle surface potential and second-order surface susceptibility from SHS data for membranes with different content of lipids with phosphate headgroups, and observe composition dependent behaviour which suggests structural membrane reorganization. We continue by exploring protein-membrane interactions using water as a contrast agent. We study a pore-forming toxin, perfringolysin O (PFO), known for selective insertion into cholesterol-rich membranes. Combining SHS measurements and cryo-electron microscopy, we compare the interaction between PFO and LUV membranes with a high and low cholesterol content. By obtaining a sub-picomolar insertion constant of PFO, we show that SHS can be used to determine protein-membrane binding constants in concentration ranges not accessible by conventional methods.Finally, we investigate the interaction of protein aerolysin and its mutants with lipid membranes. We observe changes in surface charge in the cap region of membrane-bound aerolysin at different pH of the surrounding solution. By combining SHS with single-channel measurements, we obtain the membrane binding constant of wild-type aerolysin and its mutants, and we dissect their pore-forming mechanism.

EPFL2022

Développement de méthodes de modélisation des flux turbulents dans un environnement alpin en utilisant des réseaux de stations micrométéorologiques

Romain Mage

Les caractéristiques spatiales et temporelles générales associées aux circulations atmosphériques induites par des effets thermiques locaux des terrains montagneux sont bien étudiées et le comportement général des systèmes de vents diurnes de montagne (vents catabatiques et anabatiques de vallée et de pente) est globalement bien compris. Toutefois, les détails liés à l'impact de l'hétérogénéité spatiale ne sont pas aussi bien compris et étudiés, en particulier l’effet de l’hétérogénéité spatiale et temporelle des circulations associé à un relief complexe sur les flux de surface à une échelle local. Dans ce travail sont présentés des résultats issus d’une campagne hydrologique menée durant l’été 2009 dans le Val Ferret, près du col du Grand Saint-Bernard dans les Alpes suisses sont présentés. Les données issues d’un réseau de 12 stations micrométéorologiques sans fil (stations Sensorscope) ainsi que d’une station de mesure de corrélation turbulente sont présentées, incluant des mesures des flux turbulents de chaleur sensible et de chaleur latente. Sur la base des résultats, deux modèles de flux turbulents à la surface ont été développés puis appliqués aux stations Sensorscope. L’objectif est de pouvoir fournir de bonnes estimations des flux turbulents de chaleur sensible et latente ainsi que de l’évapotranspiration dans un environnement alpin en utilisant uniquement des mesures simples issues de réseaux de stations micrométéorologiques sans fil, faciles à déployer dans des terrains montagneux et permettant un suivi des mesures. Cela a été fait à travers une extension du modèle de Penman-Monteith. La comparaison avec les mesures de corrélation turbulente a indiqué un haut niveau de corrélation avec le nouveau modèle (0.97 pour la chaleur latente et 0.95 pour la chaleur sensible).

2010