Desorption energy of soft particles from a fluid interface

The efficiency of soft particles to stabilize emulsions is examined by measuring their desorption free energy, i.e., the mechanical work required to detach the particle from a fluid interface. Here, we consider rubber-like elastic as well as microgel particles, using coarse-grained molecular dynamics simulations. The energy of desorption is computed for two and three-dimensional configurations by means of the mean thermodynamic integration method. It is shown that the softness affects the particle-interface binding in two opposing directions as compared to rigid particles. On the one hand, a soft particle spreads at the interface and thereby removes a larger unfavorable liquid-liquid contact area compared to rigid particles. On the other hand, softness provides the particle with an additional degree of freedom to get reshaped instead of deforming the interface, resulting in a smaller restoring force during the detachment. It is shown that the first effect prevails so that a soft spherical particle attaches to the fluid interface more strongly than rigid spheres. Finally, we consider microgel particles both in the swollen and in the collapsed state. Surprisingly, we find that the latter has a larger binding energy. All results are rationalised using thermodynamic arguments and thereby offer detailed insights into the desorption energy of soft particles from fluid interfaces.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Soft particles at a fluid interface

Particles added to a fluid interface can be used as a surface stabilizer in the food, oil and cosmetic industries. As an alternative to rigid particles, it is promising to consider highly deformable particles that can adapt their conformation at the interface. In this study we compute the shapes of soft elastic particles using molecular dynamics simulations of a cross-linked polymer gel, complemented by continuum calculations based on linear elasticity. It is shown that the particle shape is not only affected by the Young's modulus of the particle, but also strongly depends on whether the gel is partially or completely wetting the fluid interface. We find that the molecular simulations for the partially wetting case are very accurately described by the continuum theory. By contrast, when the gel is completely wetting the fluid interface the linear theory breaks down and we reveal that molecular details have a strong influence on the equilibrium shape.

Estimating the bending modulus of a FtsZ bacterial-division protein filament

FtsZ, a cytoskeletal protein homologous to tubulin, is the principle constituent of the division ring in bacterial cells. It is known to have force-generating capacity in vitro and has been conjectured to be the source of the constriction force in vivo. Several models have been proposed to explain the generation of force by the Z ring. Here we re-examine data from in vitro experiments in which Z rings formed and constricted inside tubular liposomes, and we carry out image analysis on previously published data with which to better estimate important model parameters that have proven difficult to measure by direct means. We introduce a membrane-energy-based model for the dynamics of multiple Z rings moving and colliding inside a tubular liposome and a fluid model for the drag of a Z ring as it moves through the tube. Using this model, we estimate an effective membrane bending modulus of 500-700 pN nm. If we assume that FtsZ force generation is driven by hydrolysis into a highly curved conformation, we estimate the FtsZ filament bending modulus to be 310-390 pN nm(2). If we assume instead that force is generated by the non-hydrolysis-dependent intermediate curvature conformation, we find that B(f)>1400 pN nm(2). The former value sits at the lower end of the range of previously estimated values and, if correct, may raise challenges for models that rely on filament bending to generate force.

Luteinizing hormone and human chorionic gonadotropin fragment the migrating myoelectric complex in rat small intestine

We hypothesized that sex hormones may affect motility disorders because these diseases occur more often in women than in men, and symptoms often occur or worsen after ovulation. Luteinizing hormone (LH) is predominantly secreted by the anterior pituitary midway through the menstrual cycle; it results in the development of the corpus luteum. LH levels also increase after bilateral gonadectomy. LH and human chorionic gonadotropin (hCG) bind to the same receptor, but rats lack hCG. To assess how LH and hCG influence myoelectric activity of the small intestine and to test the specificity of the LH receptor, we implanted electrodes on the jejunum of female rats. LH (0.1 or 0.5 NIH units) was administered intraperitoneally to intact and gonadectomized rats and 0.5 NIH units to rats that had been both hypophysectomized and gonadectomized; intact animals were treated with 100 units USP of hCG. Recordings were made with the rats in fasted and in fed states, and their intestinal motility was analysed. The most striking effects of LH, hypophysectomy, and hCG were the same: phase III of the migrating myoelectric complex was markedly fragmented and its duration lengthened (P < 0.0001). Gonadectomy alone and gonadectomy with hypophysectomy also increased fragmentation and phase III duration (P < 0.01 or better). LH receptors respond similarly to LH and hCG, and both hormones alter myoelectric activity of the rat small intestine in comparable ways.