Complete Wetting of Pt(111) by Nanoscale Liquid Water Films

Tunable Surface Wettability via Terahertz Electrowave Controlled Vicinal Subnanoscale Water Layer

Achieving timely, reversible, and long-range remote tunability over surface wettability is highly demanded across diverse fields, including nanofluidic systems, drug delivery, and heterogeneous catalysis. Herein, using molecular dynamic simulations, we show, for the first time, a theoretical design of electrowetting to achieve remotely controllable surface wettability via using a terahertz wave. The key idea driving the design is the unique terahertz collective vibration identified in the vicinal subnanoscale water layer, which is absent in bulk water, enabling efficient energy transfer from the terahertz wave to the rotational motion of the vicinal subnanoscale water layer. Consequently, a frequency-specific alternating terahertz electric field near the critical strength can significantly affect the local hydrogen-bonding network of the contact water layer on the solid surface, thereby achieving tunable surface wettability.

Different platinum crystal surfaces show very distinct protein denaturation capabilities

Different platinum (Pt) surfaces of nanocrystals usually exhibit significant distinctions with regard to various biological, physical, and chemical characteristics, such as bio-recognition, surface wetting, and catalytic activities. In this study, we report for the first time that two shape-controlled Pt nanocrystals with the most common low-index surfaces, Pt(100) and Pt(111), show very dissimilar protein denaturation capabilities based on all-atom molecular dynamics simulations employing the widely used model protein, villin headpiece (HP35). We demonstrate that HP35 is well preserved on the Pt(100) crystal surface, whereas it is severely disrupted on the Pt(111) crystal surface. This surprising difference originates from the distinct water behavior in the first solvation shell (FSS) of the two Pt crystal surfaces. Within the FSS of the Pt(100) crystal surface, water molecules form a very compact and stable monolayer through a highly uniform rhombic hydrogen-bond network. This water monolayer prefers the adsorption of acidic residues (such as Glu and Asp) and acts as a shield to prevent other residues from directly coming into contact with the metal surface. On the other hand, the hydrogen bond network in the water monolayer in the FSS of the Pt(111) crystal surface is very sparse and quite defective, which makes it more vulnerable to the penetration of various residues, particularly those with planar side chains such as Phe, Trp and Arg due to strong dispersion interactions, leading to subsequent protein unfolding. The binding free energy calculations for some key amino acids on the two different crystal surfaces further uncover the molecular origin behind their distinct protein denaturation capability. Our study reveals the vital importance of interfacial water in determining the structure of proteins when binding to different metal crystal surfaces. The discovered molecular mechanisms may be helpful for the future development of a bio-assisted programmable synthetic strategy of sophisticated Pt nanostructures for biomedical applications.

Temperature regulation of the contact angle of water droplets on the solid surfaces

We investigate theoretically the stability of the wetting property, i.e., the contact angle values, as a function of the temperature. We find that the estimated temperature coefficient of the contact angle for the water droplets on an ordered water monolayer on a 100 surface of face-center cubic (FCC) is about one order of magnitude larger than that on a hydrophobic hexagonal surface in the temperature range between 290 K and 350 K, using molecular dynamics simulations. As temperature rises, the number of hydrogen bonds between the ordered water monolayer and the water droplet will increase, which therefore enhances the hydrophilicity of the ordered water monolayer at the FCC model surface. Our work thus provides an easily controllable and reversible way to control the degree of hydrophobicity of various solid surfaces exhibiting a similar wetting property of water droplets on the ordered water monolayer as such particular FCC (100) surfaces.

Nanoscale Structure and Dynamics of Water on Pt and Cu Surfaces from MD Simulations

The interaction of liquid water with Pt(111) is investigated with classical molecular dynamics (MD) simulations, where the forces are determined using the third-generation charge optimized many-body (COMB3) interatomic potential. In cases of sub-monolayer water coverage, the parameterized empirical potential predicts experimentally observed and energetically favorable √37 and √39 reconstructed water structures with "575757" di-interstitial defects. At both sub-monolayer and multilayer water coverages, the structure of the first wetting layer of liquid water on Pt(111) exhibits a characteristic distribution where the molecules form two distinct buckled layers as a result of the interplay between water-metal adsorption and water-water hydrogen bonds. The dynamic spreading rate of water nanodroplets on large Pt surfaces (>200 nm²) characterized by molecular kinetic spreading theory is an order of magnitude slower than the molecular kinetic rate of the same droplet on close-packed Cu surfaces due to variation in molecular distributions at the water-metal interface. These nanoscale MD simulation predictions using the COMB3 interatomic potential demonstrate the capability of capturing both many-body interactions between H₂O and Pt or Cu and hydrogen bonding in liquid water.

Wettability alteration in a functional capillary tube for visual quantitative point of care testing

Capillarity is an extremely common physical-chemical phenomenon related to wettability in nature, which has wide theoretical and practical interest. Herein, we reported a facile sensing device based on capillary force change in a vertical capillary tube. In this height-based capillary sensor (HCS), the inner surface of the capillary tube was modified with a layer of molecules with wetting responsibility based on the well-known simple surface chemistry. With targets in different concentrations, the wettability of the surface modified with responsive molecules would produce different changes. The responsive surfaces would change the capillary force of the vertical capillary tube, and result in different column heights. Like a thermometer, H⁺ and phenol have been quantified visually based on the height of the liquid inside the capillary tube.

Reversible Hydrophobicity-Hydrophilicity Transition Modulated by Surface Curvature

Wettability (hydrophobicity and hydrophilicity) is of fundamental importance in physical, chemical, and biological behaviors, resulting in widespread interest. Herein, by modulating surface curvature, we observed a reversible hydrophobic-hydrophilic transition on a model referred to a platinum surface. The underlying mechanism is revealed to be the competition between strong water-solid attraction and interfacial water orderliness. On the basis of the competition, we further propose an equation of wetting transition in the presence of an ordered interfacial liquid. It quantitatively reveals the relation of solid wettability with interfacial water orderliness and solid surface curvature, which can be used for predicting the critical point of the wetting transition. Our findings thus provide an innovative perspective on the design of a functional device demonstrating a reversible wettability transition and even a molecular-level understanding of biological functions.

Homogeneous Nucleation of Ice in Transiently-Heated, Supercooled Liquid Water Films

We have investigated the nucleation and growth of crystalline ice in 0.24 μm thick, supercooled water films adsorbed on Pt(111). The films were transiently heated with ∼10 ns infrared laser pulses, which produced typical heating and cooling rates of ∼10⁹-10¹⁰ K/s. The crystallization of these water films was monitored with infrared spectroscopy. The experimental conditions were chosen to suppress ice nucleation at both the water/metal and water/vacuum interfaces. Furthermore, internal pressure increases due to curvature effects are precluded in these flat films. Therefore, the experiments were sensitive to the homogeneous ice nucleation rate from ∼210 to 225 K. The experiments show that J_max, the maximum for the homogeneous ice nucleation rate, J(T), needs to be ≥10²⁶ m^-3 s^-1 and is likely to be ∼10^29±2 m^-3 s^-1. We argue that such large nucleation rates are consistent with experiments on hyperquenched glassy water, which typically have crystalline fractions of ∼1% or more.

Direct Prediction of Calcite Surface Wettability with First-Principles Quantum Simulation

Prediction of intrinsic surface wettability from first-principles offers great opportunities in probing new physics of natural phenomena and enhancing energy production or transport efficiency. We propose a general quantum mechanical approach to predict the macroscopic wettability of any solid crystal surfaces for different liquids directly through atomic-level density functional simulation. As a benchmark, the wetting characteristics of calcite crystal (10.4) under different types of fluids (water, hexane, and mercury), including either contact angle or spreading coefficient, are predicted and further validated with experimental measurements. A unique feature of our approach lies in its capability of capturing the interactions among various polar fluid molecules and solid surface ions, particularly their charge density difference distributions. Moreover, this approach provides insightful and quantitative predictions of complicated surface wettability alteration problems and wetting behaviors of liquid/liquid/solid triphase systems.

A nanosecond pulsed laser heating system for studying liquid and supercooled liquid films in ultrahigh vacuum

A pulsed laser heating system has been developed that enables investigations of the dynamics and kinetics of nanoscale liquid films and liquid/solid interfaces on the nanosecond time scale in ultrahigh vacuum (UHV). Details of the design, implementation, and characterization of a nanosecond pulsed laser system for transiently heating nanoscale films are described. Nanosecond pulses from a Nd:YAG laser are used to rapidly heat thin films of adsorbed water or other volatile materials on a clean, well-characterized Pt(111) crystal in UHV. Heating rates of ∼10(10) K/s for temperature increases of ∼100-200 K are obtained. Subsequent rapid cooling (∼5 × 10(9) K/s) quenches the film, permitting in-situ, post-heating analysis using a variety of surface science techniques. Lateral variations in the laser pulse energy are ∼±2.7% leading to a temperature uncertainty of ∼±4.4 K for a temperature jump of 200 K. Initial experiments with the apparatus demonstrate that crystalline ice films initially held at 90 K can be rapidly transformed into liquid water films with T > 273 K. No discernable recrystallization occurs during the rapid cooling back to cryogenic temperatures. In contrast, amorphous solid water films heated below the melting point rapidly crystallize. The nanosecond pulsed laser heating system can prepare nanoscale liquid and supercooled liquid films that persist for nanoseconds per heat pulse in an UHV environment, enabling experimental studies of a wide range of phenomena in liquids and at liquid/solid interfaces.

A nonmonotonic dependence of the contact angles on the surface polarity for a model solid surface

We found an unusual nonmonotonic contact angle dependence of the surface polarity (denoted as q) on a solid surface with specific charge patterns, where the contact angle firstly decreases and then increases as q increases from 0 e to 1.0 e.