Dynamic Behavior of a Nanosized Water Droplet on the Stepped Surface with a Wetting Gradient.

The present study investigated the dynamic behavior of a nanosized water droplet on a flat and stepped surface using molecular dynamics simulations. The effects of a wetting gradient associated with the surface and the step height of a stepped surface on the dynamic behavior of the water droplet were considered in this study. The dynamic behaviors of the water droplet were described quantitatively upon analyzing the transient variation of the adhesion energy and the position of the water droplet along with the time required to climb the step. The water droplet moved smoothly along the surface with an increasing wetting gradient. On the other hand, the step obstructed the water droplet from climbing the step as the step height increases. The dynamic behavior of the water droplet depending on the variation of the normalized step height and the differences in adhesion energies between the different surfaces was classified into three types, namely, (1) fully climbing the step, (2) partially climbing the step, and (3) being blocked by the step. In the case in which the water droplet fully climbs the step, the time taken for the water droplet to fully climb the step showed a non-monotonic pattern as the step height increases.

Molecular Insights on the Wetting Behavior of a Surface Corrugated with Nanoscale Domed Pillars.

Using all-atom molecular dynamics simulation, we investigated the wettability of a surface texturized with nanoscale pillars of domed, rectangular, or cylindrical shapes. The dewetted and wetted states of the gaps between the pillars were related to the Cassie-Baxter (CB) and Wenzel (WZ) states of a macroscopic water droplet resting on top of the pillars. We uncovered the structures and free energies of the intermediate states existing between the CB and WZ states. The contact line of the liquid-vapor-solid interface could not be depinned for the domed pillars due to their smooth curvatures unlike for the rectangular or cylindrical pillars. The liquid symmetrically penetrated down into the gap between the domed pillars by a liquid-vapor interface shape like a paraboloid, while the penetration for the rectangular or cylindrical pillars was often asymmetrical, giving a half-tubular liquid-vapor interface.

Wetting Behavior of a Surface with Dual-Scale Structures.

Theoretical and numerical studies were conducted to investigate the transitional interpillar spacing for dual-scale structures, where wetting transition between the Wenzel and Cassie-Baxter states occurs in the primary and secondary pillars. A theoretical formula was derived for the transitional interpillar spacing based on the continuum picture of water. Molecular dynamics (MD) simulations were carried out by varying the interpillar spacing for the primary pillars for single- and dual-scale structures with various pillar heights. The results obtained from the theoretical formula agreed reasonably well with the results obtained from MD simulations, especially when the primary pillar height was relatively high. The transitional interpillar spacing increases as the pillar height and the number of secondary pillars increase. The effect of the secondary pillars on the transitional interpillar spacing was also evaluated using the difference in the grand potentials between the Wenzel and Cassie-Baxter states. These results show that the dual-scale structures increase the transitional interpillar spacing with an increase in the surface hydrophobicity.

Molecular Dynamics Study of the Hydrophilic-to-Hydrophobic Switching in the Wettability of a Gold Surface Corrugated with Spherical Cavities.

This paper reports a large scale molecular dynamics (MD) simulation study of the wettability of a gold surface engraved with (hemi)spherical cavities. By increasing the depth of cavities, the contact angle (CA) of a water droplet on the surface was varied from a hydrophilic (69°) to a hydrophobic value (>109°). The nonmonotonic behavior of the CA vs the depth of the cavities was consistent with the Cassie-Baxter theory, as found in the experiment by Abdelsalam et al. (Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Wetting of Regularly Structured Gold Surfaces. Langmuir 2005, 21, 1753-1757). Depending on the depth of cavities, however, the droplet existed not only in the Cassie-Baxter state, but also in the Wenzel or an intermediate state, where the cavities were penetrated partially by the droplet.

Molecular dynamics study on the wettability of a hydrophobic surface textured with nanoscale pillars.

Using molecular dynamics simulation, we studied the wetting properties of a surface textured with hydrophobic pillars, several nanometers in size. The drying transition of water confined between square or circular pillars was related to the Wenzel (WZ) to Cassie-Baxter (CB) transition of a water droplet deposited on periodic pillars. The inter-pillar spacing at which the drying occurs was compared to that predicted from the continuum theory. Such a comparison revealed that the line tension plays an important role in the drying behavior of the present nm-sized pillars. The water molecules near the pillar walls were layered and ordered in orientation. Our simulation showed a long-lived CB state which eventually turns into the WZ state. In this transition, water slowly penetrated down into the inter-pillar gap until it reached the half height of the pillar, and then quickly reached the base of the pillar.

Dynamic behavior of water droplets on solid surfaces with pillar-type nanostructures.

In the present study, we investigated the static and dynamic behavior of water droplets on solid surfaces featuring pillar-type nanostructures by using molecular dynamics simulations. We carried out the computation in two stages. As a result of the first computational stage, an initial water cube reached an equilibrium state at which the water droplet showed different shapes depending on the height and the lateral and gap dimensions of the pillars. In the second computational stage, we applied a constant body force to the static water droplet obtained from the first computational stage and evaluated the dynamic behavior of the water droplet as it slid along the pillar-type surface. The dynamic behavior of the water droplet, which could be classified into three different groups, depended on the static state of the water droplet, the pillar characteristics (e.g., height and the lateral and gap dimensions of the pillars), and the magnitude of the applied body force. We obtained the advancing and receding contact angles and the corresponding contact angle hysteresis of the water droplets, which helped classify the water droplets into the three different groups.

Application of virtual product design to the development of HVAC solution for Incheon International Airport Modular COVID-19 testing center.

Owing to the spread of COVID-19, the need for an inspection center that can quickly determine whether travelers using the airport are infected has emerged. For rapid determination, not only polymerase chain reaction tests but also antigen-antibody tests and on-site analysis systems are required. However, because it is time- and cost-intensive to construct a building that meets the standards for negative pressure facilities, modular negative pressure facilities are being installed as alternatives. Existing negative pressure facilities have problems such as increased energy consumption due to outdoor air load and condensation due to differences in indoor and outdoor temperatures and humidities caused by excessive external air inflow to achieve the target negative pressure and air change rate (ACH). In addition, owing to the installation of additional devices, additional construction is required to use them for other purposes in the future. To solve these problems, in this study, energy recovery ventilation (ERV) was employed to develop a heating, ventilation and air conditioning (HVAC) solution for the Incheon International Airport COVID-19 Testing Center. To shorten the development period, virtual product design (VPD) using computational fluid dynamics analysis-based design of experiments was performed. Owing to the application of VPD, the Incheon International Airport Modular COVID-19 Testing Center was completed in 2 weeks. The target pressure was measured in all spaces by applying the optimal conditions derived through VPD. In addition, owing to the application of ERV, the ACH of an airborne infectious isolation room exceeded the value suggested by international organizations.

A study on the behavior of water droplet confined between an atomic force microscope tip and rough surfaces.

The atomic force microscope (AFM) is used for imaging, measuring, and manipulating matter at the nanoscale. It is well-known that water condenses between an AFM tip and a solid surface, thereby generating a pull-off force acting on the tip. We investigated the behavior of a water meniscus between the tip and a solid surface using molecular dynamics simulation. We considered ideally smooth surfaces and rough surfaces that are regularly structured and randomly generated with a standard deviation of 2 A. The characteristic energy values of the solid surfaces used in the study are 0.1, 0.5, 1.0, 1.5, 2.0, and 2.5 kcal/mol, and the tip-to-surface distance considered is in the range from 1.5 to 3.7 nm. The behavior of water confined between the tip and a solid surface depends on the characteristic energy of the solid surface, the tip-to-surface distance, and the shape of the solid surface. The contact angle, neck radius of the water meniscus, and absolute value of capillary force decreases as the tip-to-surface distance increases, regardless of the pattern of the solid surface. Compared to an ideally smooth surface, the effect of regularly structured roughness on the behavior of a water meniscus on a solid surface is significant, whereas the effect of randomly generated roughness is relatively small.

Contrasting water adhesion strengths of hydrophobic surfaces engraved with hierarchical grooves: lotus leaf and rose petal effects.

The (de)wetting transitions of hierarchical grooves periodically engraved on a hydrophobic surface were investigated using a fully atomistic molecular dynamics simulation. The (meta) stable and transition states with sagging or depinning liquid surfaces were identified by calculating the free energy profiles of the (de)wetting transitions. The dewetting transitions for wide and narrow minor grooves have large and small activation free energies, respectively, exhibiting contrasting water adhesion forces as found for rose petals and lotus leaves.

Direct numerical simulation of a near-field particle-laden plane turbulent jet.

The present study investigates the gas-particle two-phase flow in the turbulent plane jet by solving the two-dimensional and compressible flow fields numerically using direct-numerical-simulation technique. The flow fields are spatially developing, but we focus our study on the evolution of coherent vortex structures and dispersion patterns of particles in the near field at different Stokes numbers. The initial symmetric mode of flow changes its shape to the asymmetric mode after about three convection periods as the flow moves downstream. The concessive paring processes between two and three vortex structures are observed. The predicted mean velocity profiles show self-similar behavior and coincide well with previous experimental data. The profiles of turbulent intensity and Reynolds shear stress also display self-similar characteristics in the further downstream regions. The local-focusing phenomena of particles occur in the quasicoherent dispersion structure of particles. The higher density distribution at the outer boundary of large-scale vortex structure characterizes the dispersion pattern of particles at the Stokes numbers of order of unity. Furthermore, these particles disperse largely along the lateral direction and show the nonuniform distribution of concentration. For the particles at the Stokes number of 0.01, the dispersion along the lateral direction is considerable due to the small aerodynamics response time, but the particles are distributed evenly in the flow field. Particles at the Stokes numbers of 10 and 50 disperse much less along the lateral direction with the even density distribution. These results support the previous conclusions on the dispersion of particles in the free shear flows.

Static and dynamic contact angles of water droplet on a solid surface using molecular dynamics simulation.

The present study investigates the variation of static contact angle of a water droplet in equilibrium with a solid surface in the absence of a body force and the dynamic contact angles of water droplet moving on a solid surface for different characteristic energies using the molecular dynamics simulation. With increasing characteristic energy, the static contact angle in equilibrium with a solid surface in the absence of a body force decreases because the hydrophobic surface changes its characteristics to the hydrophilic surface. In order to consider the effect of moving water droplet on the dynamic contact angles, we apply the constant acceleration to an individual oxygen and hydrogen atom. In the presence of a body force, the water droplet changes its shape with larger advancing contact angle than the receding angle. The dynamic contact angles are compared with the static contact angle in order to see the effect of the presence of a body force.