Gravity on Earth limits the study of the properties of pure fluids near critical point because they become stratified under their own weight. Near the critical point, all thermodynamic properties either diverge or converge and the heating and cooling cause instabilities of the convective flow as a consequence of the expansibility divergence. In order to study boiling, fluctuation and phase separation processes near the critical point of pure fluids without the influence of the Earth's gravity, a number of experiments were performed in the weightlessness of Mir space station. The experimental setup called ALICE II instrument was designed to suppress sedimentation and buoyancy-driven flow. Another set of experiments were carried out on Earth using a carefully density matched system of deuterated methanolcycloxexane to observe critical fluctuations directly. The set of experiments performed on board of Mir space station studied boiling and wetting film dynamics during evaporation near the critical point of two pure fluids (sulfur hexafluoride and carbon dioxide) using a defocused grid method. The specially designed cell containing the pure fluid was heated and, as a result, a low contrast line appeared on the wetting film that corresponded to a sharp change in the thickness of the film. A large mechanical response was observed in response to the cell heating and we present quantitative results about the receding contact lines. It is found that the vapor recoil force is responsible for the receding contact line. Local density fluctuations were observed by illuminating a cylindrical cell filled with the pure fluid near its liquid- gas critical point and recorded using a microscope and a video recorder. Microscopic fluctuations were analyzed both in sulfur hexafluoride and in a binary mixture of methanol cyclohexane. Using image processing techniques, we were able to estimate the properties of the fluid from the recorded images showing fluctuations of the transmitted and scattered light. We found that the histogram of an image can be fitted to a Gaussian relationship and by determining its width we were able to estimate the position of the critical point. The characteristic length of the fluctuations corresponding to the maximum of the radial average of the power spectrum was also estimated. The power law growth for the early stage of the phase separation was determined for two different temperature quenches in pure fluid and these results are in agreement with other experimental results and computational simulations.

This study investigated the critical behaviour of ternary mixtures of DOPC and DPPC, with cholesterol. The properties of model membranes such as these are studied in order to provide insight into aspects of complex biological systems. Experiments were performed using the Jeener echo, a static solid-state NMR technique, however no information about the critical phenomena was obtained. Conversely, the sideband linewidths measured from 2H MAS NMR are sensitive to temperature and dependent upon the phase behaviour. By fitting the linewidth data to an equation from Suwelack et al. (J. Chem. Phys., 1980; 73(6):2559-2569), the critical temperature and the critical exponent for the correlation length of the system were calculated. The critical exponent values obtained from these samples ranged between νc = 0.65 and νc = 1.2, which encompasses the critical exponents for both the 2D and 3D Ising models within error. The universality class for these model membranes cannot be unambiguously assigned yet.

A theoretical explanation of the existence of lipid rafts in cell membranes remains a topic of lively debate. Large, micrometer sized rafts are readily observed in artificial membranes and can be explained using thermodynamic models for phase separation and coarsening. In live cells such domains are not observed and various models are proposed to describe why the systems do not coarsen. We review these attempts critically and show within a phase field approach that membrane bound proteins have the potential to explain the different behaviour observed in vitro and in vivo. Large scale simulations are performed to compute scaling laws and size distribution functions under the influence of membrane bound proteins and to observe a significant slow down of the domain coarsening at longer times and a breakdown of the self-similarity of the size-distribution function. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

A theoretical explanation of the existence of lipid rafts in cell membranes remains a topic of lively debate. Large, micrometer sized rafts are readily observed in artificial membranes and can be explained using thermodynamic models for phase separation and coarsening. In live cells such domains are not observed and various models are proposed to describe why the systems do not coarsen. We review these attempts critically and show within a phase field approach that membrane bound proteins have the potential to explain the different behaviour observed in vitro and in vivo. Large scale simulations are performed to compute scaling laws and size distribution functions under the influence of membrane bound proteins and to observe a significant slow down of the domain coarsening at longer times and a breakdown of the self-similarity of the size-distribution function. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

This work addresses properties of two-component lipid membranes on the experimentally relevant spatial scales of order of a micrometer and time intervals of order of a second by means of lattice-based Monte Carlo (MC) simulations. To be able to do that with reasonable computational efforts the lipid membrane is modeled as a square lattice of lipid molecules with next-neighbor interaction. This allows for efficient computation and thus provides a large-scale simulation with which it was possible to obtain important results previously not reported in simulation studies of lipid membranes. After properly tuning the next-neighbor interaction energies the simulation reproduces the experimental phase diagram of the DMPC/DSPC lipid system which is used as a model system in this work. Beyond that, the MC simulation provides a more detailed description of the phase behavior of the lipid mixture than the experimental data. It is found that, within a certain range of lipid compositions, the phase transition from the fluid phase to the fluid–gel phase coexistence proceeds via near-critical fluctuations, while for other lipid compositions this phase transition has a quasi-abrupt character. The complete combined state and component phase diagram is constructed by structure function analysis which confirms the existence of a critical point in the system. The dynamics of membrane coarsening after an abrupt temperature quench to the fluid–gel coexistence region of the phase diagram are studied. In this context, it is found that lateral diffusion of lipids plays an important role in the fluid–gel phase separation process. Dynamic scaling is observed only if the ratio of gel and fluid phase in the membrane stays constant in time. The line tension characterizing lipid domains in the fluid–gel coexistence region is found to be in the pN range thus matching values both predicted theoretically and measured experimentally. When approaching the critical point, the line tension, the inverse correlation length of fluid–gel spatial fluctuations, and the corresponding inverse order parameter susceptibility of the membrane vanish in agreement with recent experimental findings for model lipid membranes. By simulating single particle tracking and fluorescence correlation spectroscopy experiments it is found that in the presence of near-critical fluctuations lipid molecules show transient subdiffusive behavior, which is a new result important for understanding the origins of subdiffusion in cell membranes which are believed to be close to a critical point. The membrane–cytoskeleton interaction strongly affects phase separation, enhances subdiffusion, and eventually leads to hop diffusion of lipids. Thus, a minimum realistic model for membrane rafts showing the features of both microscopic phase separation and subdiffusion is established.

This work addresses properties of two-component lipid membranes on the experimentally relevant spatial scales of order of a micrometer and time intervals of order of a second by means of lattice-based Monte Carlo (MC) simulations. To be able to do that with reasonable computational efforts the lipid membrane is modeled as a square lattice of lipid molecules with next-neighbor interaction. This allows for efficient computation and thus provides a large-scale simulation with which it was possible to obtain important results previously not reported in simulation studies of lipid membranes. After properly tuning the next-neighbor interaction energies the simulation reproduces the experimental phase diagram of the DMPC/DSPC lipid system which is used as a model system in this work. Beyond that, the MC simulation provides a more detailed description of the phase behavior of the lipid mixture than the experimental data. It is found that, within a certain range of lipid compositions, the phase transition from the fluid phase to the fluid–gel phase coexistence proceeds via near-critical fluctuations, while for other lipid compositions this phase transition has a quasi-abrupt character. The complete combined state and component phase diagram is constructed by structure function analysis which confirms the existence of a critical point in the system. The dynamics of membrane coarsening after an abrupt temperature quench to the fluid–gel coexistence region of the phase diagram are studied. In this context, it is found that lateral diffusion of lipids plays an important role in the fluid–gel phase separation process. Dynamic scaling is observed only if the ratio of gel and fluid phase in the membrane stays constant in time. The line tension characterizing lipid domains in the fluid–gel coexistence region is found to be in the pN range thus matching values both predicted theoretically and measured experimentally. When approaching the critical point, the line tension, the inverse correlation length of fluid–gel spatial fluctuations, and the corresponding inverse order parameter susceptibility of the membrane vanish in agreement with recent experimental findings for model lipid membranes. By simulating single particle tracking and fluorescence correlation spectroscopy experiments it is found that in the presence of near-critical fluctuations lipid molecules show transient subdiffusive behavior, which is a new result important for understanding the origins of subdiffusion in cell membranes which are believed to be close to a critical point. The membrane–cytoskeleton interaction strongly affects phase separation, enhances subdiffusion, and eventually leads to hop diffusion of lipids. Thus, a minimum realistic model for membrane rafts showing the features of both microscopic phase separation and subdiffusion is established.