Optimization of Patterned Surfaces for Improved Superhydrophobicity through Cost-Effective Large-Scale Computations.

Thermodynamic analysis on wetting states and wetting state transitions of rough surfaces

H.26L is an emerging video encoding standard proposed by ITU, which goal is for video communication and multicast in low bit rate conditions. We give detailed analysis and performance tests for the parallelism of the H.26L video encoder test model long-term (TML8.0). Based on these tests, several parallel schedule strategies are discussed in different aspects, and the optimal one is selected and implemented by using MPI. At last, experiment results on Dawning-2000 MPP machines with different processors are given. The analysis and simulation results show that the H.26L has a good parallel performance and scalability on slice layer, and parallel computing can greatly speed up the processing of H.26L encoder. The experiment methods and results can be applied to many parallel systems for H.26L video processing.

A new approach to fabricate ordered complex surface patterns is established upon linearly polarized light (LPL) irradiation with a continuous wave laser. Distinct from typical laser‐induced periodic surface structures, the ordered patterns are formed through unique topographical transitions of submicrometer pillar arrays of an azo molecular glass, when irradiated with LPL at 532 nm. As revealed by microscopic investigation and optical simulation, the structure formation results from the deformation of the original pillars along the electric vibration direction of LPL and growth of new structures in the regions between the pillars owing to the nonuniformly distributed light intensity. Spontaneously shaped structures are also induced by increasing LPL irradiation time. The ordered surface patterns are then formed by coalescence of the gradually erased pillars with these newly formed structures. The morphologies and orientation of the patterns are precisely controlled by adjusting the conditions, i.e., the polarization direction of LPL relative to the hexagonal lattice of the pillar arrays and the irradiation time. These observations lead to the deep understanding of unique light–matter interaction effects on the material in pillar array architecture. This new method can be used to create various complex surface patterns in submicrometer scale for many applications.

Periodic submiocrometer pillar arrays are fabricated from a photoresponsive azo molecular glass (IA‐Chol) by soft‐lithographic hot embossing with elastomeric poly(dimethylsiloxane) (PDMS) modes. Through deformation of each submicrometer‐sized pillar in response to the local amplitude and polarization of the superimposed electric waves, optical holograms are recorded on the IA‐Chol pillar arrays. When the interference pattern is formed by two polarized beams with opposite‐circular polarizations, the recorded patterns mainly reflect the polarization state variations with spatial phase difference of the interfering waves. When two plane waves with the same linear polarizations are superimposed, where the polarization direction is almost the same as the writing beams, the intensity variation of the superimposed electric waves is recorded by the pillar arrays changing spatially with the phase variations. Various ordered surface patterns with distinct hierarchical configurations are successfully developed by the intensity and polarization modulations of the interfering waves. This approach not only allows to directly visualize the intensity and polarization of the coherent light captured by the holograms, but also provides a powerful platform to fabricate various complex surface patterns. The submicrometer pillar arrays can also be used to record polarization hologram and the images are reconstructed by reference light in diffracted spots.

A facile electrochemical anodization method was used for producing hierarchically textured surfaces based on TiO2 nanotubes in two different configurations. It was found that perfluoro-functionalized TiO2 nanotubes exhibit high static contact angles for a variety of liquids such as apolar, polar aprotic and polar protic solvents. Wenzel and Cassie-Baxter theories were applied for theoretical contact angle calculations for the present study. By using Cassie theories, it is shown that a drop of polar liquid was in a fakir or Cassie-Baxter (CB) state on perfluoro-functionalized nanotube surfaces. The fakir state prevents spreading of the liquid on the surface. On the other hand, the wetting of non-polar liquids such as hexane is characterized by either Wenzel states or transition states characterized by partial imbibition that lie in between the CB and Wenzel states.

Ab initio calculations indicated that icosahedral structure is more stable for B12Hn+ clusters with n = 6–12, but planar configuration is more stable for B12Hn+ with n = 0–5. Here, we report the energy barriers of structure transition by detaching H atoms from icosahedral B12H6+ to planar B12H5+, planar B12H4+, icosahedral B12H5+ or icosahedral B12H4+ cluster. We performed density functional theory calculations to explore the energy barriers and the reaction path of the structure transition. The resultant energy barriers are almost the same, enabling the transition to proceed and suggesting that the structure of B12Hn+ clusters is controlled by the number of hydrogen atoms n.

Initiation mechanisms of enhanced pyrolysis and oxidation of JP-10 (exo-tetrahydrodicyclopentadiene) on functionalized graphene sheets: Insights from ReaxFF molecular dynamics simulations

The 1,3-dipolar cycloaddition reactions of diazomethane with ethylene and formaldehyde as well as the nitrogen elimination reactions from the cycloadducts have been studied using density functional and conventional ab initio methods. The exothermicity of the reactions is underestimated by DFT methods with respect to CCSD(T) due to an overestimation of the C−N dissociation energy of diazomethane. For the cycloaddition reactions all methods lead to similar transition state geometries, and the potential energy barriers computed using DFT methods are similar to the CCSD(T) ones. On the other hand, for the nitrogen elimination reactions transition state geometries and energy barriers are more dependent on the level of calculation. The results obtained show that for the reaction between diazomethane and ethylene the pyrazoline intermediate is more stable than the reactants and that the Gibbs energy barrier for nitrogen elimination is larger than the barrier corresponding to its formation. On the contrary, for formaldehyde the kinetically most favorable cycloadduct, 1,2,3-oxadiazoline, is less stable than the reactants and has a lower barrier for nitrogen elimination.

We study the wetting transition on microstructured hydrophobic surfaces. We use the string method [J. Chem. Phys. 2007, 126, 164103; J. Chem. Phys. 2013, 138, 134105] to accurately compute the transition states, the energy barriers, and the minimum energy paths for the wetting transition from the Cassie-Baxter state to the Wenzel state. Numerical results are obtained for the wetting of a hydrophobic surface textured with a square lattice of pillars. It is found that the wetting of the solid substrate occurs via infiltration of the liquid in a single groove, followed by lateral propagation of the liquid front. The propagation of the liquid front proceeds in a stepwise manner, and a zipping mechanism is observed during the infiltration of each layer. The minimum energy path for the wetting transition goes through a sequence of intermediate metastable states, whose wetted areas reflect the microstructure of the patterned surface. We also study the dependence of the energy barrier on the drop size and the gap between the pillars.

A phase-field crystal model is used to investigate the mechanisms of formation and growth of early clusters in quenched/aged dilute binary alloys, a phenomenon typically outside the scope of molecular dynamics time scales. We show that formation of early subcritical clusters is triggered by the stress relaxation effect of quenched-in defects, such as dislocations, on the energy barrier and the critical size for nucleation. In particular, through analysis of system energetics, we demonstrate that the growth of subcritical clusters into overcritical sizes occurs due to the fact that highly strained areas in the lattice locally reduce or even eliminate the free energy barrier for a first-order transition.

This review provides a comprehensive description of the self-guided Langevin dynamics (SGLD) and the self-guided molecular dynamics (SGMD) methods and their applications. Example systems are included to provide guidance on optimal application of these methods in simulation studies. SGMD/SGLD has enhanced ability to overcome energy barriers and accelerate rare events to affordable time scales. It has been demonstrated that with moderate parameters, SGLD can routinely cross energy barriers of 20 kT at a rate that molecular dynamics (MD) or Langevin dynamics (LD) crosses 10 kT barriers. The core of these methods is the use of local averages of forces and momenta in a direct manner that can preserve the canonical ensemble. The use of such local averages results in methods where low frequency motion "borrows" energy from high frequency degrees of freedom when a barrier is approached and then returns that excess energy after a barrier is crossed. This self-guiding effect also results in an accelerated diffusion to enhance conformational sampling efficiency. The resulting ensemble with SGLD deviates in a small way from the canonical ensemble, and that deviation can be corrected with either an on-the-fly or a post processing reweighting procedure that provides an excellent canonical ensemble for systems with a limited number of accelerated degrees of freedom. Since reweighting procedures are generally not size extensive, a newer method, SGLDfp, uses local averages of both momenta and forces to preserve the ensemble without reweighting. The SGLDfp approach is size extensive and can be used to accelerate low frequency motion in large systems, or in systems with explicit solvent where solvent diffusion is also to be enhanced. Since these methods are direct and straightforward, they can be used in conjunction with many other sampling methods or free energy methods by simply replacing the integration of degrees of freedom that are normally sampled by MD or LD.

Microfabrication techniques allow the development and production of artificial superhydrophobic surfaces that possess a precisely controlled roughness at the micrometer level, typically achieved through the arrangement of micropillar structures in periodic patterns. In this work, we analyze the stability and energy barrier of droplets in the Cassie-Baxter (CB) state on such periodic patterns. In addition, we further develop a transition criterion using the CB equation and derive an improved version which allows predicting for which pillar geometries, equilibrium contact angles, and droplet volumes the CB state switches from a metastable to an unstable state. This enables a comparison with existing experiments and three-dimensional multiphase Lattice Boltzmann simulations for different pillar distances, two contact angles, and two droplet volumes, where a good agreement has been found.

As one of the most fundamental processes, excited-state proton transfer (ESPT) plays a major role in both chemical and biological systems. In the past several decades, experimental and theoretical studies on ESPT systems have attracted considerable attention because of their tremendous potential in fluorescent probes, biological imaging, white-light-emitting materials, and organic optoelectronic materials. ESPT is related to fluorescence properties and usually occurs on an ultrafast time scale at or below 100 fs. Consequently, steady-state and femtosecond time-resolved absorption, fluorescence, and vibrational spectra have been used to explore the mechanism of ESPT. However, based on previous experimental studies, direct information, such as transition state geometries, energy barrier, and potential energy surface (PES) of the ESPT reaction, is difficult to obtain. These data are important for unravelling the detailed mechanism of ESPT reaction and can be obtained from state-of-the-art ab initio excited-state calculations. In recent years, an increasing number of experimental and theoretical studies on the detailed mechanism of ESPT systems have led to tremendous progress. This Account presents the recent advances in theoretical studies, mainly those from our group. We focus on the cases where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of excited-state double proton transfer (ESDPT), revealing the sensing mechanism of ESPT chemosensors, illustrating the effect of intermolecular hydrogen bonding on the excited-state intramolecular proton transfer (ESIPT) reaction, investigating the fluorescence quenching mechanism of ESPT systems by twisting process, and determining the size of the solute·(solvent) n cluster for the solvent-assisted ESPT reaction. Through calculation of vertical excitation energies, optimization of excited-state geometries, and construction of PES of the ESPT reactions, we provide modifications to experimentally proposed mechanisms or completely new mechanism. Our proposed new and inspirational mechanisms based on theoretical studies can successfully explain the previous experimental results; some of the mechanisms have been further confirmed by experimental studies and provided guidance for researchers to design new ESPT chemosensors. Determination of the energy barrier from an accurate PES is the key to explore the ESPT mechanism with theoretical methods. This approach becomes complicated when the charge transfer state is involved for time-dependent density functional theory (TDDFT) method and optimally tuned range-separated TDDFT provides an alternative way. To unveil the driving force of ESPT reaction, the excited-state molecular dynamics combined with the intrinsic reaction coordinate calculations can be employed. These advanced approaches should be used for further studies on ESPT systems.

Pyramidal II to basal transformation of ⟨c + a⟩ edge dislocations in Mg-Y alloys

A co-located partitions strategy for parallel CFD–DEM couplings