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Since Shi et al. proposed that the climate in the drylands of Northwest China experienced a significant transition from a “warming and drying” trend to a “warming and wetting” trend in the 1980s, researchers have conducted numerous studies on the variations in precipitation and humidity in the region and even in arid Central Asia. In particular, the process of the “warming and wetting” trend by using obtained measurement data received much attention. However, there remain uncertainties about whether the “warming and wetting” trend has paused and what its future variations may be. In this study, we examined the spatiotemporal variations in temperature, precipitation, the aridity index (AI), vegetation, and runoff during 1950–2019. The results showed that the climate in the drylands of Northwest China and the northern Tibetan Plateau is persistently warming and wetting since the 1980s, with an acceleration since the 1990s. The precipitation/humidity variations in North China, which are mainly influenced by summer monsoon, are generally opposite to those in the drylands of Northwest China. This reverse change is mainly controlled by an anomalous anticyclone over Mongolia, which leads to an anomalous easterly wind, reduced water vapor output, and increased precipitation in the drylands of Northwest China. While it also causes an anomalous descending motion, increased water vapor divergence, and decreased precipitation in North China. Precipitation is the primary controlling factor of humidity, which ultimately forms the spatiotemporal pattern of the “westerlies-dominated climatic regime” of antiphase precipitation/humidity variations between the drylands of Northwest China and monsoonal region of North China. The primary reasons behind the debate of the “warming and wetting” trend in Northwest China were due to the use of different time series lengths, regional ranges, and humidity indices in previous analyses. Since the EC-Earth3 has a good performance for simulating precipitation and humidity in Northwest and North China. By using its simulated results, we found a wetting trend in the drylands of Northwest China under low emission scenarios, but the climate will gradually transition to a “warming and drying” trend as emissions increase. This study suggests that moderate warming can be beneficial for improving the ecological environment in the drylands of Northwest China, while precipitation and humidity in monsoon-dominated North China will persistently increase under scenarios of increased emissions.

Wetting, the process of water interacting with a surface, is critical in our everyday lives and in many biological and technological systems. The contact angle is the angle at the interface where water, air and solid meet, and its value is a measure of how likely the surface is to be wetted by the water. Low contact-angle values demonstrate a tendency of the water to spread and adhere to the surface, whereas high contact-angle values show the surface’s tendency to repel water. The most common method for surface-wetting characterization is sessile-drop goniometry, due to its simplicity. The method determines the contact angle from the shape of the droplet and can be applied to a wide variety of materials, from biological surfaces to polymers, metals, ceramics, minerals and so on. The apparent simplicity of the method is misleading, however, and obtaining meaningful results requires minimization of random and systematic errors. This article provides a protocol for performing reliable and reproducible measurements of the advancing contact angle (ACA) and the receding contact angle (RCA) by slowly increasing and reducing the volume of a probe drop, respectively. One pair of ACA and RCA measurements takes ~15–20 min to complete, whereas the whole protocol with repeat measurements may take ~1–2 h. This protocol focuses on using water as a probe liquid, and advice is given on how it can be modified for the use of other probe liquids.

This study investigates a novel microfluidic mixing technique that uses the resonant oscillation of coalescent droplets. During the vertical contact-separation process, solutes are initially separated as a result of the combined effects of diffusion and gravity. We show that the application of alternating current (AC) voltage to microelectrodes below the droplets causes a resonant oscillation, which enhances the even distribution of the solute. The difference in concentration between the top and bottom droplets exhibits frequency dependence and indicates the existence of a particular AC frequency that results in a homogeneous concentration. This frequency corresponds to the resonance frequency of the droplet oscillation that is determined using particle tracking velocimetry. To understand the mixing process, a phenomenological model based on the equilibrium between surface tension, viscosity, and electrostatic force was developed. This model accurately predicted the resonance frequency of droplet flow and was consistent with the experimental results. These results suggest that the resonant oscillation of droplets driven by AC voltage significantly enhances the diffusion of solutes, which is an effective approach to microfluid mixing.

When a droplet comes in contact with a completely wetting surface, the liquid typically spreads until it covers the entire substrate. However, nonuniform evaporation of a multi-component droplet can generate surface tension gradients that alter this behavior. Here, we explore the rich dynamics of fully miscible, three-component droplets composed of water, ethanol, and propylene glycol on completely wetting glass substrates. These droplets initially spread rapidly but then stop and contract. We experimentally and theoretically investigate this behavior throughout the ternary parameter space at different relative humidities. Evaporation changes the composition of the droplet over space and time, resulting in a reversal of Marangoni flows that ultimately determines the dynamic droplet shape. We illustrate the utility of such dynamics by collecting, aggregating, and removing contaminants from a 4-cm² area using a single μL-scale droplet.

A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar’s circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water–air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.

We focus on the competition between nitrate/nitrite ammonification (also termed dissimilatory nitrate reduction to ammonium (DNRA)) and denitrification processes taking place across partially saturated water‐soil systems. The study is motivated by the observation that the joint presence of dissolved organic nitrogen (DON) and redox fluctuation in the vadose zone poses potential risks for generation of nitrates (NO3−‐N) that can then be reduced to ammonium (NH4+‐N) through DNRA. We examine nitrogen dynamics induced in natural soil samples subject to controlled drying‐wetting cycles. Upon experimental evidences, we estimate the parameters driving the kinetics associated with nitrogen transformation. This enables us to document a competition between DNRA and denitrification during wetting periods. We find that the increasing the carbon‐to‐nitrogen (C/N) ratio in the system yields a significant increase of DNRA rates, with a corresponding increase of their contribution to nitrate reduction. The rate of DNRA is documented to be (a) significantly faster in loam than in sandy loam, due to dissolved carbon release from loam aggregates, and (b) more effective in the presence of amino acid than urea in the natural soil, due to the role of amino acid as carbon source. Our analysis further suggests the relevance of hydrogeochemical factors (e.g., moisture variation, soil texture, and C/N ratio) on DON transformation through the influence of functional microorganisms. These insights advance our understanding of nitrogen dynamics in agroecosystems, which has significant implications for environmental management practices aimed at controlling NO3−‐N pollution in partially saturated soils. Key Points Competition between denitrification and dissimilatory nitrate reduction to ammonium (DNRA) is identified in transformation of organic nitrogen Contribution of DNRA to nitrate reduction depends on soil texture, C/N ratio and nitrogen speciation Ignoring the occurrence of DNRA may underestimate nitrate leaching through vadose zone

Since the pioneering prediction of surface melting by Michael Faraday, it has been widely accepted that thin water layers, called quasi-liquid layers (QLLs), homogeneously and completely wet ice surfaces. Contrary to this conventional wisdom, here we both theoretically and experimentally demonstrate that QLLs have more than two wetting states and that there is a first-order wetting transition between them. Furthermore, we find that QLLs are born not only under supersaturated conditions, as recently reported, but also at undersaturation, but QLLs are absent at equilibrium. This means that QLLs are a metastable transient state formed through vapor growth and sublimation of ice, casting a serious doubt on the conventional understanding presupposing the spontaneous formation of QLLs in ice–vapor equilibrium. We propose a simple but general physical model that consistently explains these aspects of surface melting and QLLs. Our model shows that a unique interfacial potential solely controls both the wetting and thermodynamic behavior of QLLs.

In living cells, protein-rich condensates can wet the cell membrane and surfaces of membrane-bound organelles. Interestingly, many phase-separating proteins also bind to membranes leading to a molecular layer of bound molecules. Here we investigate how binding to membranes affects wetting, prewetting and surface phase transitions. We derive a thermodynamic theory for a three-dimensional bulk in the presence of a two-dimensional, flat membrane. At phase coexistence, we find that membrane binding facilitates complete wetting and thus lowers the wetting angle. Moreover, below the saturation concentration, binding facilitates the formation of a thick layer at the membrane and thereby shifts the prewetting phase transition far below the saturation concentration. The distinction between bound and unbound molecules near the surface leads to a large variety of surface states and complex surface phase diagrams with a rich topology of phase transitions. Our work suggests that surface phase transitions combined with molecular binding represent a versatile mechanism to control the formation of protein-rich domains at intra-cellular surfaces.