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

Investigation of Flow and Heat Transfer Performance of Double-Layer Pin-Fin Manifold Microchannel Heat Sinks

The manifold microchannel (MMC) heat sink is characterized by high heat transfer efficiency, high compactness, and low flow resistance. It can be an effective method for the high-flux removal of high-power electronic components. To further enhance the performance of the MMC, a double-layer pin–fin MMC structure was designed. The thermodynamic properties, including the flow and heat transfer characteristics, were numerically investigated using ANSYS Fluent with deionized water as the working liquid. Compared with the single-layer MMC, the temperature uniformity is better, the pressure drop is lower, and the comprehensive performance is improved at the cost of slightly larger thermal resistance for the double-layer MMC. The geometric effects on the thermodynamic performance were also analyzed. The results show that among the pin–fin structures with round, diamond-shaped, and rectangular cross-sections, the round pin–fins demonstrate the best comprehensive performance and the minimal thermal resistance. Under the same inlet velocity, the thermal resistance is decreased, and the comprehensive performance is first increased and then decreased as the pin–fin size increases. In addition, it is recommended to adopt a larger height ratio for low inlet velocity and a smaller height ratio for high inlet velocity

Solvent Exchange Leading to Nanobubble Nucleation: A Molecular Dynamics Study

The solvent exchange procedure has become the most-used protocol to produce surface nanobubbles, while the molecular mechanisms behind the solvent exchange are far from being fully understood. In this paper, we build a simple model and use molecular dynamics simulations to investigate the dynamic characteristics of solvent exchange for producing nanobubbles. We find that at the first stage of solvent exchange, there exists an interface between interchanging solvents of different gas solubility. This interface moves toward the substrate gradually as the exchange process proceeds. Our simulations reveal directed diffusion of gas molecules against the gas concentration gradient, driven by the solubility gradient of the liquid composition across the moving solvent-solvent interface. It is this directed diffusion that causes gas retention and produces a local gas oversaturation much higher near the substrate than far from it. At the second stage of solvent exchange, the high local gas oversaturation leads to bubble nucleation either on the solid surface or in the bulk solution, which is found to depend on the substrate hydrophobicity and the degree of local gas oversaturation. Our findings suggest that solvent exchange could be developed into a standard procedure to produce oversaturation and used to a variety of nucleation applications other than generating nanobubbles

Controllable Preparation and Direct Observation of an Interconversion between Kinetically and Thermodynamically Stable Luminescent Multicuprous Coordination Complexes

We report the controlled preparation of kinetically and thermodynamically luminescent multinuclear cuprous coordination complexes. On reaction of the bidentate P/N-ligand (2-(diphenylphosphino)pyridine, abbreviated as dppy) and copper(I) with a similar stoichiometry but in different concentration conditions at ambient temperature, hexagonal crystals of [Cu2(μ-dppy)3Cl]PF6·CH2Cl2 (1) or polyhedron crystals of [Cu4(μ-dppy)6Cl](PF6)3 (2) were formed as kinetic or thermodynamic products, respectively. Investigation of the structures, morphologies, theoretical calculation, and photophysical properties of the multinuclear cuprous coordination complexes demonstrates the formation of and differences between kinetically and thermodynamically stable conformations. Single-crystal X-ray structural analyses indicated that 1 features the head-to-head orientation dppy bridged dinuclear trefoil-like Cu2-core with one terminal Cl–, and tetranuclear 2 can be regarded as a Cl– bridged dimer of 1 with two-third flipping head-to-tail dppy ligands. The density functional theory (DFT) calculations demonstrated that 2 had a lower frontier molecular orbital energy than 1, with an energy difference of 320.76 kJ/mol. Intriguingly, the controllable reversible transformation between 1 and 2 can be achieved under suitable conditions. By soaking the crystal in the mother liquor or alcohol, the kinetically stable 1 can slowly transform to the thermodynamically stable 2, or conversely, 2 can also quickly reconstruct to 1 when it is soaked in a chloride solution. Steadily changing morphologies, photoluminescence, as well as powder X-ray diffraction patterns were observed during the gradual transformation process. Based on structural analyses, a feasible transforming mechanism was proposed. The energetic and structural analyses of and the interconversion process between kinetically and thermodynamically stable multicuprous complexes here provide insights into the relations among the structures, morphologies, and properties of the metal–organic coordination materials at the molecular level

Charged nanochannels endow COF membrane with weakly concentration-dependent methanol permeability

Covalent organic frameworks (COFs) with long-range ordered, rigid, and tailor-made nanochannels hold great promise for energy-related applications. Here, we fabricate COF membranes with one-dimensional charged nanochannels using ionic COF nanosheets decorated with sulfonic acid groups. The free-standing and robust COF membrane exhibits enhanced proton conductivity and suppressed methanol permeability compared to the benchmark Nafion membrane. More interestingly, the methanol permeability of the COF membrane remains almost constant in a wide range of methanol concentrations (5.35 x 10(-7) cm(2) s(-1) for the 2 M methanol/water mixture and 5.44 x 10(-7) cm(2 & nbsp;& nbsp;)s(-1) for the neat methanol). All-atomistic dynamics simulations indicate that the diffusio-osmotic effect arising from the charged nanochannels can account for the low and nearly constant methanol permeability of the COF membranes. These findings suggest that the ionic COF membranes can find potential applications in direct methanol fuel cell with high methanol concentrations, and meanwhile shed light on the mass transport mechanism in the charged, rigid and ordered nanochannels

Charged nanochannels endow COF membrane with weakly concentration-dependent methanol permeability

Covalent organic frameworks (COFs) with long-range ordered, rigid, and tailor-made nanochannels hold great promise for energy-related applications. Here, we fabricate COF membranes with one-dimensional charged nanochannels using ionic COF nanosheets decorated with sulfonic acid groups. The free-standing and robust COF membrane exhibits enhanced proton conductivity and suppressed methanol permeability compared to the benchmark Nafion membrane. More interestingly, the methanol permeability of the COF membrane remains almost constant in a wide range of methanol concentrations (5.35 x 10(-7) cm(2) s(-1) for the 2 M methanol/water mixture and 5.44 x 10(-7) cm(2 & nbsp;& nbsp;)s(-1) for the neat methanol). All-atomistic dynamics simulations indicate that the diffusio-osmotic effect arising from the charged nanochannels can account for the low and nearly constant methanol permeability of the COF membranes. These findings suggest that the ionic COF membranes can find potential applications in direct methanol fuel cell with high methanol concentrations, and meanwhile shed light on the mass transport mechanism in the charged, rigid and ordered nanochannels