Thin water films grown at ambient conditions on BaF2(111) studied by scanning polarization force microscopy

Water-Controlled Structural Transition and Charge Transfer of Interfacial Ionic Liquids

Clarification of the water-induced structural transitions and electron transfer between ionic liquids (ILs) and a solid surface allows for establishing a unified view of the electrical properties of interfacial ILs via a hitherto unexplored pathway. Here, we propose a simple and effective method to quantitatively identify and extract the transferred electrons between ILs and a solid surface, while demonstrating the critical structural transition of interfacial ILs from ordered stripe structures to disordered aggregation structures. The formation of hydrated anions, rooted in the hydrogen bonds of O-H···O between the anion and water, lies at the tipping point where electron transfer ends and aggregation structure begins. In addition, it is discovered to what extent the hydrophilicity of substrates can affect electron transfer, and a regulation method based on the electric field is explored. These experimental findings may refresh our knowledge of interfacial ILs and provide an effective method for evaluating the intrinsic electrical features of the ILs-solid surface.

Studying Ice with Environmental Scanning Electron Microscopy

Scanning electron microscopy (SEM) is a powerful imaging technique able to obtain astonishing images of the micro- and the nano-world. Unfortunately, the technique has been limited to vacuum conditions for many years. In the last decades, the ability to introduce water vapor into the SEM chamber and still collect the electrons by the detector, combined with the temperature control of the sample, has enabled the study of ice at nanoscale. Astounding images of hexagonal ice crystals suddenly became real. Since these first images were produced, several studies have been focusing their interest on using SEM to study ice nucleation, morphology, thaw, etc. In this paper, we want to review the different investigations devoted to this goal that have been conducted in recent years in the literature and the kind of information, beyond images, that was obtained. We focus our attention on studies trying to clarify the mechanisms of ice nucleation and those devoted to the study of ice dynamics. We also discuss these findings to elucidate the present and future of SEM applied to this field.

Freezing efficiency of feldspars is affected by their history of previous freeze-thaw events

Among the different aerosol mineral particles that contribute to induce ice nucleation (IN) in the troposphere, feldspars have been identified as the most active. Nevertheless, which surface properties make some feldspars more efficient than others, i.e. able to induce IN at higher temperatures, is still unclear. In addition to that, surface properties of such materials can change as they are exposed to a variety of environmental conditions while traveling through the troposphere. Here, freezing temperature of water droplets deposited on feldspar minerals has been measured as a function of consecutive freeze-thaw cycles. We found an increase of the freezing temperature for the initial cycles followed by approximately constant freezing temperature for consecutive cycles. We call this a "history effect". This effect is more evident for samples aged in standard room conditions and it disappears if the sample is exposed to oxygen plasma. Oxygen plasma generates OH groups at the surface, facilitating IN and cleans the surface from organic contamination, unblocking pores at the surface, believed to be the most active IN sites on feldspars. A similar process is suggested to happen during the history effect, when consecutive freeze-thaw events unblock IN sites.

The Effect of Adsorbed Volatile Organic Compounds on an Ultrathin Water Film Measurement

Using surface plasmon resonance imaging (SPRi), we have recently shown for the first time the existence of a monolayer water film between droplets during dropwise condensation. This study examines the effect of adsorbed volatile organic compounds (VOCs) on the ultrathin film measurement using SPRi. Further, the work presents the proper surface-treatment process that enables measurements of the ultrathin water layer during high-speed imaging of dropwise condensation at 3000 frame per second. In this study, two methods were applied for cleaning the surface (gold-coated glass)—(1) standard cleaning procedure (SCP) using acetone, isopropyl alcohol, and deionized water and (2) SCP followed by air plasma cleaning. This work discusses the effect of the cleaning procedures on surface roughness, contact angle, and surface chemistry using atomic force microscopy, optical microscopy, and an X-ray photoelectron spectroscope meter. The results showed that SCP before the SPRi is a proper surface-treatment method. The effect of adsorbed VOCs during dropwise condensation on a surface treated with SCP was measured to be 0.0025 (reflectivity unit), which was 70% smaller than the reflectance associated with a monolayer water film. The results of this work confirm a monolayer water film observation during the dropwise condensation, which has been reported before.

Nanoscale dielectric microscopy of non-planar samples by lift-mode electrostatic force microscopy

Lift-mode electrostatic force microscopy (EFM) is one of the most convenient imaging modes to study the local dielectric properties of non-planar samples. Here we present the quantitative analysis of this imaging mode. We introduce a method to quantify and subtract the topographic crosstalk from the lift-mode EFM images, and a 3D numerical approach that allows for extracting the local dielectric constant with nanoscale spatial resolution free from topographic artifacts. We demonstrate this procedure by measuring the dielectric properties of micropatterned SiO2 pillars and of single bacteria cells, thus illustrating the wide applicability of our approach from materials science to biology.

Imaging Water Thin Films in Ambient Conditions Using Atomic Force Microscopy

All surfaces exposed to ambient conditions are covered by a thin film of water. Other than at high humidity conditions, i.e., relative humidity higher than 80%, those water films have nanoscale thickness. Nevertheless, even the thinnest film can profoundly affect the physical and chemical properties of the substrate. Information on the structure of these water films can be obtained from spectroscopic techniques based on photons, but these usually have poor lateral resolution. When information with nanometer resolution in the three dimensions is needed, for example for surfaces showing heterogeneity in water affinity at the nanoscale, Atomic Force Microscopy (AFM) is the preferred tool since it can provide such resolution while being operated in ambient conditions. A complication in the interpretation of the data arises when using AFM, however, since, in most cases, direct interaction between a solid probe and a solid surface occurs. This induces strong perturbations of the liquid by the probe that should be controlled or avoided. The aim of this review is to provide an overview of different AFM methods developed to overcome this problem, measuring different interactions between the AFM probe and the water films, and to discuss the type of information about the water film that can be obtained from these interactions.

Nanoscale electric polarizability of ultrathin biolayers on insulating substrates by electrostatic force microscopy

We measured and quantified the local electric polarization properties of ultrathin (∼5 nm) biolayers on mm-thick mica substrates. We achieved it by scanning a sharp conductive tip (<10 nm radius) of an electrostatic force microscope over the biolayers and quantifying sub-picoNewton electric polarization forces with a sharp-tip model implemented using finite-element numerical calculations. We obtained relative dielectric constants εr = 3.3, 2.4 and 1.9 for bacteriorhodopsin, dioleoylphosphatidylcholine (DOPC) and cholesterol layers, chosen as representative of the main cell membrane components, with an error below 10% and a spatial resolution down to ∼50 nm. The ability of using insulating substrates common in biophysics research, such as mica or glass, instead of metallic substrates, offers both a general platform to determine the dielectric properties of biolayers and a wider compatibility with other characterization techniques, such as optical microscopy. This opens up new possibilities for biolayer research at the nanoscale, including nanoscale label-free composition mapping.

Highly compressed two-dimensional form of water at ambient conditions

The structure of thin-film water on a BaF₂(111) surface under ambient conditions was studied using x-ray absorption spectroscopy from ambient to supercooled temperatures at relative humidity up to 95%. No hexagonal ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The oxygen K-edge x-ray absorption spectrum of liquid thin-film water on BaF₂ exhibits, at all temperatures, a strong resemblance to that of high-density phases for which the observed spectroscopic features correlate linearly with the density. Surprisingly, the highly compressed, high-density thin-film liquid water is found to be stable from ambient (300 K) to supercooled (259 K) temperatures, although a lower-density liquid would be expected at supercooled conditions. Molecular dynamics simulations indicate that the first layer water on BaF₂(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.

The microscopic structure of adsorbed water on hydrophobic surfaces under ambient conditions

The interaction of water vapor with hydrophobic surfaces is poorly understood. We utilize graphene templating to preserve and visualize the microscopic structures of adsorbed water on hydrophobic surfaces. Three well-defined surfaces [H-Si(111), graphite, and functionalized mica] were investigated, and water was found to adsorb as nanodroplets (∼10-100 nm in size) on all three surfaces under ambient conditions. The adsorbed nanodroplets were closely associated with atomic-scale surface defects and step-edges and wetted all the hydrophobic substrates with contact angles<∼10°, resulting in total water adsorption that was similar to what is found for hydrophilic surfaces. These results point to the significant differences between surface processes at the atomic/nanometer scales and in the macroscopic world.

Measuring the true height of water films on surfaces

Measuring the level of hydrophilicity of heterogeneous surfaces and the true height of water layers that form on them in hydrated conditions has a myriad of applications in a wide range of scientific and technological fields. Here, we describe a true non-contact mode of operation of atomic force microscopy in ambient conditions and a method to establish the source of apparent height. A dependency of the measured water height on operational parameters is identified with water perturbations due to uncontrolled modes of imaging where intermittent contact with the water layer, or even the surface, might occur. In this paper we show how to (1) determine when the water is being perturbed and (2) distinguish between four different interaction regimes. Each of the four types of interaction produces measurements ranging from fractions of the true height in one extreme to values which are as large as four times the real height in the other. We show the dependence of apparent height on the interaction regime both theoretically and empirically. The agreement between theory and experiment on a BaF2(111) sample displaying wet and un-wet regions validates our results.

Influence of the macroscopic shape of the tip on the contrast in scanning polarization force microscopy images

We demonstrate that a quantitative analysis of the contrast obtained in electrostatic force microscopy images that probe the dielectric response of the sample (scanning polarization force microscopy (SPFM)) requires numerical simulations that take into account both the macroscopic shape of the tip and the nanoscopic tip apex. To simulate the SPFM contrast, we have used the generalized image charge method (GICM), which is able to accurately deal with distances between a few nanometers and several microns, thus involving more than three orders of magnitude. Our numerical simulations show that the macroscopic shape of the tip accounts for most of the SPFM contrast. Moreover, we find a quasi-linear relation between the working tip-sample distance and the contrast for tip radii between 50 and 200 nm. Our calculations are compared with experimental measurements of the contrast between a thermally grown silicon oxide sample and a few-layer graphene film transferred onto it.