A Bright First Day for Tidal Disruption Event

Stream-stream collision may be an important pre-peak energy dissipation mechanism in tidal disruption events (TDEs). We perform local three-dimensional radiation hydrodynamic simulations in a wedge geometry including the gravity to study stream self-crossing, with emphasis on resolving the collision and following the subsequent outflow. We find that the collision can contribute to pre-peak optical emissions by converting $\gtrsim5\%$ of stream kinetic energy to radiation, yielding prompt emission of $\sim10^{42-44}\rm erg~s^{-1}$. The radiative efficiency is sensitive to stream mass fallback rates, and strongly depends on the downstream gas optical depth. Even for a sub-Eddington ($10\%$) mass fallback rate, the strong radiation pressure produced in the collision can form a local super-Eddington region near the collision site, where a fast, aspherical outflow is launched. Higher mass fallback rate usually leads to more optically-thick outflow and lower net radiative efficiency. For $\dot{M}\gtrsim0.1\dot{M}_{\rm Edd}$, the estimated photosphere size of the outflow can expand by one to two orders of magnitudes reaching $\sim10^{14}\rm cm$. The average gas temperature at this photospheric surface is a few $\times10^{4}$K, roughly consistent with inferred pre-peak photosphere properties for some optical TDEs. We find that the dynamics is sensitive to collision angle and collision radius, but the radiative efficiency or outflow properties show more complex dependency than is often assumed in ballistic models.Comment: 18 pages, 18 figures, Submitted to ApJ. Comments are welcomed and appreciated

Pore-scale numerical simulation of spontaneous imbibition in porous media containing fractures

Spontaneous imbibition is an essential mechanism for recovering oil from low-permeability fractured water-driven reservoirs. To accurately capture the migration interface of oil-water two-phase flow under these conditions, this study employs phase field theory coupled with Cahn-Hilliard and Navier-Stokes equations. We conduct a numerical pore-scale investigation on countercurrent imbibition in low-permeability fractured porous media. The results show that pore-scale spontaneous imbibition can be divided into four stages. In the first stage, oil-water film is formed when oil contacts with water, and this contact line moves under the action of capillary force. In the second stage, the oil film at the end of the oil cluster ruptures to form isolated oil droplets. In the third stage, these oil droplets are surrounded by water and gradually transported outward. In the final stage, oil droplets accumulate in the fractures and are collectively expelled from the matrix. In the process of oil droplet migration, the phenomenon of sticking occurs under the influence of water extrusion and the internal structure of the matrix, which leads to the formation of residual oil. The increased complexity of open boundary and fracture development strengthens the imbibition effect by elevating the degree of spontaneous imbibition pore utilization, thus improving the oil utilization efficiency. The above findings can provide a numerical modeling reference for the study of spontaneous imbibition in fractured porous media, and at the same time, has some guiding significance for the development of low-permeability reservoirs.Document Type: Original articleCited as: Yang, L., Jiang, X., Li, M., Gong, F., Dong, G., Li, X. Pore-scale numerical simulation of spontaneous imbibition in porous media containing fractures. Capillarity, 2024, 10(2): 48-56. https://doi.org/10.46690/capi.2024.02.0

The regulator of G protein signaling (RGS) domain of G protein-coupled receptor kinase 5 (GRK5) regulates plasma membrane localization and function.

The G protein-coupled receptor (GPCR) kinases (GRKs) phosphorylate activated GPCRs at the plasma membrane (PM). Here GRK5/GRK4 chimeras and point mutations in GRK5 identify a short sequence within the regulator of G protein signaling (RGS) domain in GRK5 that is critical for GRK5 PM localization. This region of the RGS domain of GRK5 coincides with a region of GRK6 and GRK1 shown to form a hydrophobic dimeric interface (HDI) in crystal structures. Coimmunoprecipitation (coIP) and acceptor photobleaching fluorescence resonance energy transfer assays show that expressed GRK5 self-associates in cells, whereas GRK5-M165E/F166E (GRK5-EE), containing hydrophilic mutations in the HDI region of the RGS domain, displays greatly decreased coIP interactions. Both forcing dimerization of GRK5-EE, via fusion to leucine zipper motifs, and appending an extra C-terminal membrane-binding region to GRK5-EE (GRK5-EE-CT) recover PM localization. In addition, GRK5-EE displays a decreased ability to inhibit PAR1-induced calcium release compared with GRK5 wild type (wt). In contrast, PM-localized GRK5-EE-CaaX (appending a C-terminal prenylation and polybasic motif from K-ras) or GRK5-EE-CT shows comparable ability to GRK5 wt to inhibit PAR1-induced calcium release. The results suggest a novel model in which GRK5 dimerization is important for its plasma membrane localization and function

Growth diagram of La0.7Sr0.3MnO3 thin films using pulsed laser deposition

An experimental study was conducted on controlling the growth mode of La0.7Sr0.3MnO3 thin films on SrTiO3 substrates using pulsed laser deposition (PLD) by tuning growth temperature, pressure and laser fluence. Different thin film morphology, crystallinity and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the adatom nucleation, step advance processes and their relationship to film growth were theoretically analyzed and a growth diagram was constructed. Three boundaries between highly and poorly crystallized growth, 2D and 3D growth, stoichiometric and non-stoichiometric growth were identified in the growth diagram. A good fit of our experimental observation with the growth diagram was found. This case study demonstrates that a more comprehensive understanding of the growth mode in PLD is possible

Performance studies of a SiPM-readout system with a pico-second timing chip

A pico-second timing (PIST) front-end electronic chip has been developed using $55~\mathrm{nm}$ CMOS technology for future electron-positron collider experiments (namely Higgs factories). Extensive tests have been performed to evaluate the timing performance of a dedicated SiPM-readout system equipped with a PIST chip. The results show that the system timing resolution can achieve $45~\mathrm{ps}$ for SiPM signals at the minimum-ionizing particles (MIP) level ($200~\mathrm{p.e.}$) and better than $10~\mathrm{ps}$ for signals larger than $1200~\mathrm{p.e.}$, while the PIST intrinsic timing resolution is $4.76 \pm 0.60~\mathrm{ps}$. The PIST dynamic range has been further extended using the time-over-threshold (ToT) technique, which can cover the SiPM response spanning from $\mathrm{\sim 900~p.e.}$ to $~\mathrm{\sim 40000~p.e.}$

Indications of magnetic coupling effects in spin cross-over molecular thin films

Room temperature isothermal reversible spin crossover switching of [Fe(H2B(pz)2)2(bipy)] thin films is demonstrated. The magnetic oxide substrate locks the [Fe{H2B(pz)2}2(bipy)] largely in a low spin state. With an X-ray fluence, excitation to a high spin state occurs, while relaxation back to low spin state is aided by alternating the substrate magnetization. Includes supplementary materials