Analyzing the Molecular Kinetics of Water Spreading on Hydrophobic Surfaces via Molecular Dynamics Simulation

Molecular Dynamics Investigation of Nanoscale Hydrophobicity of Polymer Surfaces: What Makes Water Wet?

The wettability of a polymer surface─related to its hydrophobicity or tendency to repel water─can be crucial for determining its utility, such as for a coating or a purification membrane. While wettability is commonly associated with the macroscopic measurement of a contact angle between surface, water, and air, the molecular physics that underlie these macroscopic observations are not fully known, and anticipating the relative behavior of different polymers is challenging. To address this gap in molecular-level understanding, we use molecular dynamics simulations to investigate and contrast interactions of water with six chemically distinct polymers: polytetrafluoroethylene, polyethylene, polyvinyl chloride, poly(methyl methacrylate), Nylon-66, and poly(vinyl alcohol). We show that several prospective quantitative metrics for hydrophobicity agree well with experimental contact angles. Moreover, the behavior of water in proximity to these polymer surfaces can be distinguished with analysis of interfacial water dynamics, extent of hydrogen bonding, and molecular orientation─even when macroscopic measures of hydrophobicity are similar. The predominant factor dictating wettability is found to be the extent of hydrogen bonding between polymer and water, but the precise manifestation of hydrogen bonding and its impact on surface water structure varies. In the absence of hydrogen bonding, other molecular interactions and polymer mechanics control hydrophobic ordering. These results provide new insights into how polymer chemistry specifically impacts water-polymer interactions and translates to surface hydrophobicity. Such factors may facilitate the design or processing of polymer surfaces to achieve targeted wetting behavior, and presented analyses can be useful in studying the interfacial physics of other systems.

Molecular Insights into the Effect of Crystal Planes on Droplet Wetting

Dynamic wetting by liquids on solid surfaces depends on several aspects such as surface energy, roughness, and interfacial tension, among many others. Copper (Cu), gold (Au), aluminum (Al), and silicon (Si) are a few of the most important metals that are used extensively as substrates in various industrial and biomedical applications. Metals are etched frequently on different crystal planes for fabrication purposes. Etching exposes distinct crystal planes that may come in contact with the liquids when used for different applications. The interaction of the crystal planes with the liquid that comes in contact with the solid dictates the wetting behavior of the surface. This necessitates the importance of understanding how various crystal planes of the same metals behave under similar conditions. Herein, three specific crystal planes, namely, (1 0 0), (1 1 0), and (1 1 1), are investigated at a molecular scale for the above-mentioned metals. The dynamic contact angle and contact diameter trends revealed that the relatively hydrophobic surfaces (Cu, Si) tend to reach their equilibrium contact angle faster compared to the hydrophilic substrates (Al, Au). Molecular kinetic theory is used to estimate the three-phase contact line friction which is found to be higher for (1 1 1) planes. Further, a consistent potential energy distribution variation is observed for the crystal lattice of (1 0 0), (1 1 0), and (1 1 1). These findings can be used as a guideline to determine the factors needed to completely describe a dynamic wetting phenomenon of the droplet over the different crystal planes. The understanding will be of great use in deciding experimental strategies where fabricated different crystal planes would be required to have a liquid contact.

Improving the self-assembly of bioresponsive nanocarriers by engineering doped nanocarbons: a computational atomistic insight

Here, molecular dynamics (MD) simulations were employed to explore the self-assembly of polymers and docetaxel (DTX) as an anticancer drug in the presence of nitrogen, phosphorous, and boron-nitrogen incorporated graphene and fullerene. The electrostatic potential and the Gibbs free energy of the self-assembled materials were used to optimize the atomic doping percentage of the N- and P-doped formulations at 10% and 50%, respectively. Poly lactic-glycolic acid (PLGA)- polyethylene glycol (PEG)-based polymeric nanoparticles were assembled in the presence of nanocarbons in the common (corresponding to the bulk environment) and interface of organic/aqueous solutions (corresponding to the microfluidic environment). Assessment of the modeling results (e.g., size, hydrophobicity, and energy) indicated that among the nanocarbons, the N-doped graphene nanosheet in the interface method created more stable polymeric nanoparticles (PNPs). Energy analysis demonstrated that doping with nanocarbons increased the electrostatic interaction energy in the self-assembly process. On the other hand, the fullerene-based nanocarbons promoted van der Waals intramolecular interactions in the PNPs. Next, the selected N-doped graphene nanosheet was utilized to prepare nanoparticles and explore the physicochemical properties of the nanosheets in the permeation of the resultant nanoparticles through cell-based lipid bilayer membranes. In agreement with the previous results, the N-graphene assisted PNP in the interface method and was translocated into and through the cell membrane with more stable interactions. In summary, the present MD simulation results demonstrated the success of 2D graphene dopants in the nucleation and growth of PLGA-based nanoparticles for improving anticancer drug delivery to cells, establishing new promising materials and a way to assess their performance that should be further studied.

Spreading and receding of oil droplets on silanized glass surfaces in water: Role of three-phase contact line flow direction in spontaneous displacement

HYPOTHESIS

The spontaneous displacement of both spreading and receding droplets on surfaces are extensively involved in numerous technical applications. We hypothesize that the spreading and receding displacement behaviors could be interpreted differently due to opposite flow directions at the three-phase contact line.

EXPERIMENTS

We performed two groups of displacement experiments using different initial setups of oil droplets on silanized glass surfaces in aqueous surroundings.

FINDINGS

The different initial configurations mostly resulted in oil displacement in opposite directions: either spreading or receding of the oil droplet. Different static states were observed at the end of the spreading and receding processes on surfaces with the same wettability due to the contact angle hysteresis. The dynamic displacement was analyzed using the hydrodynamic and molecular kinetic models, which showed distinct applicabilities for the data description of the spreading and receding possesses. The model analysis further indicated the different nature of these possesses, in particular, the resistance to displacement dynamics, which was illustrated by the interpretation of the microscopic slip length and contact line friction in the respective models. This study can shed light on the fundamental role of the displacement direction in the spontaneous liquid-liquid displacement.

Dynamic wetting of solid-liquid-liquid system by molecular kinetic theory

HYPOTHESIS

The mechanisms of dynamic wetting of solid-liquid-liquid (SLL) system, especially the viscosity effects of two liquids, can be investigated by the molecular kinetic theory (MKT).

METHODS

The molecular kinetic theory combined with published data was used to study the roles of a fluid viscosity and a solid surface in dynamic wetting.

FINDINGS

First, the MKT on dynamic wetting was introduced and its limitation was analyzed. Second, a viscosity effect and a solid surface effect were considered. The viscosity effect was divided into three parts for the first time, including two pure liquid zones and a mixing zone. Third, a coefficient activation free energy model was proposed, considering the effects of mixing liquids and a solid surface. Finally, the key parameters in the MKT and the application and validation of the coefficient activation free energy model were discussed in detail. This model can explain the energy dissipation in a vicinity of a three-phase contact-line successfully in a SLL wetting system. This work sheds light on the physical mechanisms of fluid and solid surface properties on the dynamic wetting in a SLL system.

Evaporation of squeezed water droplets between two parallel hydrophobic/superhydrophobic surfaces

HYPOTHESIS

A liquid droplet is apt to be deformed within a compact space in various applications. The morphological change of a droplet and vapor accumulation in the confined space between two parallel surfaces with different gaps and surface wettability are expected to significantly affect the evaporation dynamics of the squeezed droplet therein.

EXPERIMENTS

Here the evaporation dynamics of a squeezed droplet between two parallel hydrophobic/superhydrophobic surfaces are experimentally explored. By reducing the surface gap from 1000 μm to 400 μm, the evolution of contact angle, contact radius and volume of the evaporating droplet are measured. A diffusion-driven model based on a two-parameter ellipsoidal segment geometry is developed to predict the morphology and volume evolution of a squeezed droplet during evaporation.

FINDINGS

Evaporation dynamics of a squeezed water droplet via the constant contact radius (CCR) mode, the constant contact angle (CCA) mode, or the mixed mode are experimentally observed. Confirmed by our ellipsoidal segment model, the evaporation of the squeezed droplet is significantly depressed with the decreasing surface gap, which is primarily attributed to vapor enrichment in a more confined geometry. A linear scaling law between droplet volume and evaporation time is unveiled, which is verified by a simplified cylindrical model.

Coalescence-Induced Swift Jumping of Nanodroplets on Curved Surfaces

In this work, we use molecular dynamics simulations to investigate coalescence-induced jumping of nanodroplets on curved surfaces with different wettabilities. On a curved surface, a liquid bridge will first form between two coalescing droplets as on a flat surface. However, contrary to symmetry-breaking-induced jumping on a flat surface, coalescing droplets would jump earlier than the liquid bridge gets into contact with the curved surface. Such an early symmetry breaking is induced by the opposite motion of coalescing droplets along the lateral direction on the curved surface. We find that surface curvature can effectively facilitate the jumping of coalescing nanodroplets via enhanced symmetry breaking. The energy conversion efficiency is improved from ∼0.15% on a flat surface to ∼2.9% on a curved surface, which is about 20 times enhancement. In addition, we conducted an energy scaling analysis by considering the lumped effects of both viscous dissipation and contact line friction on the jumping behaviors. We conclude that curvature-enhanced jumping in the nanoscale can be ascribed to the mitigated contact line dissipation E_cl, whereas viscous dissipation E_vis is maintained almost at the same level. Therefore, we unveil a scaling law between the energy conversion efficiency η on surfaces with different curvatures and the product of contact line length and contact time. Interestingly, the increasing surface curvature could allow the occurrence of coalescence-induced jumping on a less superhydrophobic surface. Hence, a phase map of coalescence-induced jumping in terms of surface curvature ratio and surface wettability is presented. Essentially, the paradigm of curved surfaces in the nanoscale used in this study is characteristic of the topography of micro/nanostructured surfaces, on which coalescence-induced droplet jumping has been experimentally observed. This work justifies the critical role of nanoroughness in boosting coalescence-induced jumping of nanodroplets and sheds light on the passive control of nanodroplets jumping on functional surfaces.

Resistant energy analysis of self-pulling process during dropwise condensation on superhydrophobic surfaces

Recently the development of superhydrophobic surfaces with one-tier or hierarchical textures has drawn increasing attention because enhanced condensation heat transfer has been observed on such biomimetic surfaces in well-tailored supersaturation or subcooling conditions. However, the physical mechanisms underlying condensation enhancement are still less understood. Here we report an energy-based analysis on the formation and growth of condensate droplets on two-tier superhydrophobic surfaces, which are fabricated by decorating carbon nanotubes (CNTs) onto microscale fluorinated pillars. Thus-formed hierarchical surfaces with two tier micro/nanoscale roughness are proved to be superior to smooth surfaces in the spatial control of condensate droplets. In particular, we focus on the self-pulling process of condensates in the partially wetting morphology (PW) from surface cavities due to intrinsic Laplace pressure gradient. In this analysis, the self-pulling process of condensate tails is resisted by adhesion energy, viscous dissipation, contact line dissipation and line tension in a combined manner. This process can be facilitated by adjusting the configuration and length scale of the first-tier texture. The optimum design can not only lower the total resistant energy but also favor the out-of-plane motion of condensate droplets anchored in the first-tier cavity. It is also shown that engineered surface with hierarchical roughness is beneficial to remarkably mitigating contact line dissipation from the perspective of molecular kinetic theory (MKT). Our study suggests that scaling down surface roughness to submicron scale can facilitate the self-propelled removal of condensate droplets.

Comparison of Collection Methods for the Measure of Human Meibum and Tear Film-Derived Lipids Using Mass Spectrometry

Purpose/aim: To assess the effectiveness of polytetrafluoroethylene (PTFE) tubes in the collection of human tears and meibum.

MATERIALS AND METHODS

This was a prospective study that enrolled 10 healthy human subjects. Both the tear film and meibum were sampled using PTFE tubes in the right eye of all subjects. In the left eyes, either 5-µL or 1-µL glass microcapillary tubes were used to collect tears, and 0.5-µL glass microcapillary tubes were used to collect meibum. The lipids from the samples were extracted and analyzed using mass spectrometry (SCIEX TripleTOF 5600, Framingham, MA, USA). The absolute peak intensities of the omega-acyl hydroxy fatty acids (OAHFA), cholesterol esters (CE), and wax esters (WE) obtained for both methods were summed and compared between collection methods.

RESULTS

A total of 10 subjects completed the study (five female, mean age: 35.7 ± 7.9 years). Using the mass spectrometer output, the median (first quartile, third quartile) summed intensity units of OAHFA, CE, and WE collected associated with tears using PTFE were 516 (125, 1315), 7946 (2571, 19,915), and 38,892 (139,630, 174,082), all of which were significantly higher (all p ≤ 0.04) than those collected from glass microcapillaries (91 (41, 408), 2463 (1389, 6042), and 11,109 (7465, 37,371), respectively). The median summed intensity units of OAHFA, CE, and WE associated with meibum (1958 (1417, 3502), 11,726 (8434, 87,691), and 84,771 (52,657, 206,050), respectively) using PTFE were not significantly different (all p ≥ 0.13) than those associated with glass microcapillaries (1502 (699, 4407), 10,781 (3287, 38,205), and 77,381 (26,590, 178,213), respectively).

CONCLUSIONS

PTFE tubes, which are thought to be lipophilic, were associated with more measurable lipids from the tear film than glass microcapillaries. There was no difference between collection methods in lipid profiles when used with meibum.

Nanoscale Water Contact Angle on Polytetrafluoroethylene Surfaces Characterized by Molecular Dynamics-Atomic Force Microscopy Imaging

The aim of this study is to link polytetrafluoroethylene (PTFE) surface characteristics with its wetting properties in the nanoscale. To do this using molecular dynamics (MD) simulation, three series of rough PTFE surfaces were generated by annealing and compressing and next characterized by the application of the MD version of the atomic force microscopy (AFM) method. The values of specific surface areas were additionally calculated. The TIP4P/2005 water model was used to study the wetting properties of obtained PTFE samples. The simulated water contact angle (WCA) value for the most flat (but slightly rough) sample having PTFE density is equal to 106.94°, and it is close to the value suggested for a perfect PTFE surface on the basis of experimental results. Also, the changes in the WCA with PTFE compression are in the same range as experimentally reported. The obtained MD simulation results make it possible to link, for the first time, the WCA values with the surface MD-AFM root-mean-square roughness and with the PTFE density. Finally, we show that for PTFE wetting in the nanoscale, the line tension is negligible and the Bormashenko's equation reduces to the Cassie-Baxter (CB) model. In fact, our simulation results are close to the CB mechanism.

The mechanism and universal scaling law of the contact line friction for the Cassie-state droplets on nanostructured ultrahydrophobic surfaces

Besides the Wenzel state, liquid droplets on micro/nanostructured surfaces can stay in the Cassie state and consequently exhibit intriguing characteristics such as a large contact angle, small contact angle hysteresis and exceptional mobility. Here we report molecular dynamics (MD) simulations of the wetting dynamics of Cassie-state water droplets on nanostructured ultrahydrophobic surfaces with an emphasis on the genesis of the contact line friction (CLF). From an ab initio perspective, CLF can be ascribed to the collective effect of solid-liquid retarding and viscous damping. Solid-liquid retarding is related to the work of adhesion, whereas viscous damping arises from the viscous force exerted on the liquid molecules within the three-phase (liquid/vapor/solid) contact zone. In this work, a universal scaling law is derived to generalize the CLF on nanostructured ultrahydrophobic surfaces. With the decreasing fraction of solid-liquid contact (i.e., the solid fraction), CLF for a Cassie-state droplet gets enhanced due to the fact that viscous damping is counter-intuitively intensified while solid-liquid retarding remains unchanged. Nevertheless, the overall friction between a Cassie-state droplet and the structured surface is indeed reduced since the air cushion formed in the interstices of the surface roughness underneath the Cassie-state droplet applies negligible resistance to the contact line. Our results have revealed the genesis of CLF from an ab initio perspective, demonstrated the effects of surface structures on a moving contact line and justified the critical role of CLF in the analysis of wetting-related situations.