RNA polymerase II clusters form in line with liquid phase wetting of chromatin

It is essential for cells to control which genes are transcribed into RNA. In eukaryotes, two major control points are recruitment of RNA polymerase II (Pol II) into a paused state and subsequent pause release to begin transcript elongation. Pol II associates with macromolecular clusters during recruitment, but it remains unclear how Pol II recruitment and pause release might affect these clusters. Here, we show that clusters exhibit morphologies that are in line with wetting of chromatin by a liquid phase enriched in recruited Pol II. Applying instantaneous structured illumination microscopy and stimulated emission double depletion microscopy to pluripotent zebrafish embryos, we find recruited Pol II associated with large clusters, and elongating Pol II with dispersed clusters. A lattice kinetic Monte Carlo model representing recruited Pol II as a liquid phase reproduced the observed cluster morphologies. In this model, chromatin is a copolymer chain containing regions that attract or repel recruited Pol II, supporting droplet formation by wetting or droplet dispersal, respectively.

Selective Separation Using Fluid-Liquid Interfaces

Interfaces between two fluid phases are a potential barrier for particles. Certain particles may not be able to pass such an interface, because they have to overcome a certain resistance. The latter depends on the strength of the interface, which is the surface tension. The second relevant property is the three phase wetting angle, which shows the fluid with the preferred wetting to the particle surface. It depends on the particle properties, like chemical composition, surface structure and surface modification. The third relevant parameter is the particle size. From these three main influence parameters it emerges that fluid-fluid interfaces can show a selectivity to special particle properties, which enables a separation of a particle mixture. Since there are possibilities to address the governing effects, the separation cut, size or composition cut respectively, can be engineered in a certain range. Separation at boundaries is feasible when the driving force is in the same order of magnitude as the retaining resistance force of the interface. The driving force is either the Brownian movement for very small particles or any field force like gravity or the centrifugal force. To describe the separation at interfaces it is necessary to understand the process of the phase transfer of particles through the interface, either the gas-liquid or the liquid-liquid interface between two immiscible liquids. In addition to the effects mentioned above, also dynamic phenomena such as surfactant depletion of the interface may have to be taken into account.

Fabrication of SiCp/ZL101 Composites Substrate with Near-Net-Shape

SiCp/Z101 composites substrate can be successfully fabricated by pressureless infiltration of ZL101 alloy liquid into porous SiCp preform, the relative density are nearly up to 99%. This is mainly because that generated SiO2phase on the surface after sintering of SiCp not only has joint function to the porous of SiC perform, but also obtains two-phase wetting between SiC and Al matrix through the interface reaction, resulting in promoting spontaneous infiltration. SiC preforms have almost no change of shapes and sizes after infiltration of Al liquid and can achieve near-net-shape of the composites for substrate. Volume fraction of SiC can be effectively improved by using binary mixture particles with the diameter ratio of 11:1, through which the properties of substrate can be controlled effctively. By the addition of SiCp, strength of the composites is improved remarkably, and its elastic modulus increases correspondingly about one time, the increase of SiC volume fraction can markedly reduce coefficient of thermal expansion (CTE) of composites , but meanwhile decrease thermal conductivity(TC) of composites, and its TC (at 50 °C ranges from 120.7 W/(m•k) to 99.4 W/(m•k) and its mean linear CTE (25°C to 50°C ) ranges from 9.47×10−6k-1to 7.05×10−6k-1as volume fraction of SiC ranges from 38% to 68%.