Hypergeometric function expressions for the molecule-micropore Lennard--Jones potential

We present hypergeometric function expressions for the molecule-micropore Lennard--Jones potential in cylindrical pores, in which the cylindrical wall can be a surface, or have thickness or have infinite thickness. These expressions are useful in theoretical study and computer simulations of fluids in micropore of circular cross sections

Robust Control Allocation among Overactuated Spacecraft Thrusters under Ellipsoidal Uncertainty

Spacecrafts with overactuated and redundant thrusters can work normally once some of them are out of work, which improves the reliability of spacecraft in orbit. In this way, the desired command of controller needs to be dynamically allocated among thrusters. Considering that uncertain factors may appear in forms of dynamics, installation errors, thrust deviations, or failures, this paper proposes a robust control allocation under ellipsoidal uncertainty. This method uses the uncertainty ellipsoid set to describe the uncertainty of the thrusters firstly and establish the thrust allocation robust reference model and then transforms it into a cone optimization model which can be solved as an optimized problem. Finally, this paper adopts the interior-point method for solving the optimization problem. In this way, difficulties of solving the problem caused by parameter uncertainties are avoided effectively. Finally, we take satellite rendezvous and docking as simulation scenarios; it is verified that the cumulative distribution error and maximum error can be reduced by more than 15% when the random error of control efficiency matrix is 5%–20%; also, precision of thruster allocation is improved

Microscopic Marangoni Flows Cannot Be Predicted on the Basis of Pressure Gradients.

A concentration gradient along a fluid-fluid interface can cause flow. On a microscopic level, this so-called Marangoni effect can be viewed as being caused by a gradient in the pressures acting on the fluid elements or as the chemical-potential gradients acting on the excess densities of different species at the interface. If the interface thickness can be ignored, all approaches should result in the same flow profile away from the interface. However, on a more microscopic scale, the different expressions result in different flow profiles, only one of which can be correct. Here we compare the results of direct nonequilibrium molecular dynamics simulations with the flows that are generated by pressure and chemical-potential gradients. We find that the approach based on the chemical-potential gradients agrees with the direct simulations, whereas the calculations based on the pressure gradients do not

What experiments on pinned nanobubbles can tell about the critical nucleus for bubble nucleation.

The process of homogeneous bubble nucleation is almost impossible to probe experimentally, except near the critical point or for liquids under large negative tension. Elsewhere in the phase diagram, the bubble nucleation barrier is so high as to be effectively insurmountable. Consequently, there is a severe lack of experimental studies of homogenous bubble nucleation under conditions of practical importance (e.g., cavitation). Here we use a simple geometric relation to show that we can obtain information about the homogeneous nucleation process from Molecular Dynamics studies of bubble formation in solvophobic nanopores on a solid surface. The free energy of pinned nanobubbles has two extrema as a function of volume: one state corresponds to a free-energy maximum ("the critical nucleus"), the other corresponds to a free-energy minimum (the metastable, pinned nanobubble). Provided that the surface tension does not depend on nanobubble curvature, the radius of the curvature of the metastable surface nanobubble is independent of the radius of the pore and is equal to the radius of the critical nucleus in homogenous bubble nucleation. This observation opens the way to probe the parameters that determine homogeneous bubble nucleation under experimentally accessible conditions, e.g. with AFM studies of metastable nanobubbles. Our theoretical analysis also indicates that a surface with pores of different sizes can be used to determine the curvature corrections to the surface tension. Our conclusions are not limited to bubble nucleation but suggest that a similar approach could be used to probe the structure of critical nuclei in crystal nucleation

Stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions

In our previous work [Phys. Chem. Chem. Phys. 2022, 24, 9685], we show with molecular dynamics simulations that bulk nanobubbles can be stabilized by forming a compressed amphiphile monolayer at bubble interfaces. This observation closely resembles the stability origin of microemulsions and inspires us to propose here that stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions: the gas-in-water nanobubble phase coexisting with the external gas phase. The stability mechanism for bulk nanobubbles is then given: The formation of compressed amphiphilic monolayer because of microbubble shrinking leads to a vanishing surface tension, and consequently the curvature energy of the monolayer dominates the thermodynamic stability of bulk nanobubbles. With the monolayer model, we further interpret several strange behaviors of bulk nanobubbles: the gas supersaturation is not a prerequisite for nanobubble stability because of the vanishing surface tension, and the typical nanobubble size of 100nm is due to the small bending constant of the monolayer. Finally, through analyzing the compressed amphiphile monolayer model we propose that bulk nanobubbles can ubiquitously exist in aqueous solutions

Elucidating Nonwetting of Re-Entrant Surfaces with Impinging Droplets

Superomniphobic surfaces display both superoleophobic and superhydrophobic properties, having a contact angle greater than 150° for both water and oil droplets. In this work, lattice Boltzmann simulations on droplets impacting the surface textures of various topologies are performed to understand the mechanism of how the superomniphobic properties can be achieved by optimizing the geometry of re-entrant surfaces and the inherent hydrophobicity of substrates. Detailed kinetics for droplet impinging is analyzed for both liquid impalement and emptying, showing distinct dependences on geometrical details of re-entrant surfaces. The origins of the enhanced stability of Cassie states are ascribed to (i) the barrier of the Cassie-to-Wenzel transition for the impalement process, (ii) the driving force for liquid receding in the emptying process, and (iii) the contact line pinning from the entrance effect and the edge effect. Finally, we check the strategies proposed here by designing a new re-entrance structure that possesses an excellent property in maintaining the droplet Cassie state

Cooperative Effect in Receptor-Mediated Endocytosis of Multiple Nanoparticles

The uptake of nanoparticles (NPs) by a cellular membrane is known to be NP size dependent, but the pathway and kinetics for the endocytosis of multiple NPs still remain ambiguous. With the aid of computer simulation techniques, we show that the internalization of multiple NPs is in fact a cooperative process. The cooperative effect, which in this work is interpreted as a result of membrane curvature mediated NP interaction, is found to depend on NP size, membrane tension, and NP concentration on the membranes. While small NPs generally cluster into a close packed aggregate on the membrane and internalize, as a whole, NPs with intermediate size tend to aggregate into a linear pearl-chain-like arrangement, and large NPs are apt to separate from each other and internalize independently. The cooperative wrapping process is also affected by the size difference between neighboring NPs. Depending on the size difference of neighboring NPs and inter-NP distance, four different internalization pathways, namely, synchronous internalization, asynchronous internalization, pinocytosis-like internalization, and independent internalization, are observed

Maximizing friction by liquid flow clogging in confinement

In the nanoscale regime, flow behaviors for liquids show qualitative deviations from bulk expectations. In this work, we reveal by molecular dynamics simulations that plug flow down to nanoscale induces molecular friction that leads to a new flow structure due to the molecular clogging of the encaged liquid. This plug-like nanoscale liquid flow shows several features differ from the macroscopic plug flow and Poiseuille flow: It leads to enhanced liquid/solid friction, producing a friction of several order of magnitude larger than that of Couette flow; the friction enhancement is sensitively dependent of the liquid column length and the wettability of the solid substrates; it leads to the local compaction of liquid molecules that may induce solidification phenomenon for a long liquid column

Computer Simulations of Solute Exchange Using Micelles by a Collision-Driven Fusion Process

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