Macroscopic Wetting Research Articles

The Impact of Wetting-Heterogeneity Distribution on Capillary Pressure and Macroscopic Measures of Wettability

Summary We investigate the effect of the length scale of wetting heterogeneities, close to the length scale of a pore, on capillary pressure saturation (CPS) curves and the United States Bureau of Mines (USBM) and Amott-Harvey (AH) wettability indices. These macroscopic wettability indices are used to describe bulk rock wettability, because the local contact angle (the standard physical measure of wettability) in a sample is difficult to access and might vary within and between pores caused by changes in mineralogy and the surface coverage of organic materials. Our study combines laboratory experiments and full-scale fluid-dynamics simulations using the multiphase stochastic-rotation dynamics (SRDmc) model. Four model systems were created using monodisperse glass beads. The surface properties of the beads were modified so that one-half of the surface area in each system was strongly hydrophilic and the other half was hydrophobic. However, each system had a different length scale of wetting heterogeneity, ranging from a fraction of a bead diameter to two bead diameters. There is excellent agreement between the experimental and simulation results. All systems are classified as intermediate-wet on the basis of their AH and USBM indices. An examination of the capillary pressure curves shows that the opening of the stable hysteresis loop decreases monotonically as the length scale of the wetting heterogeneities is increased. Thus, our results suggest that macroscopic wettability indices could be used as indicators of ultimate recovery, but they are not suited to discriminate between the different flows that occur earlier in a mixed-wettability displacement process.

Water adsorption and capillary bridge formation on silica micro-particle layers modified with perfluorinated organosilane monolayers

Monodisperse micron-sized silica particle monolayers deposited onto plasma-grown SiOx-ultra-thin films have been used as reference systems to investigate wetting, water adsorption and capillary bridge formation as a function of silica surface functionalization. 1H,1H, 2H,2H perfluorooctyltriethoxysil (FOTS) monolayers, have been deposited on the respective surfaces by means of chemical vapor deposition resulting in macroscopically low energy surfaces. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) reflection absorption spectroscopy confirmed the monolayer formation. Water adsorption isotherms were studied by a combination of in-situ FTIR reflection spectroscopy and quartz crystal microbalance (QCM) while macroscopic wetting was analysed by contact angle measurements. The comparative data evaluation indicates that the macroscopic wetting behaviour was changed as expected, however, that water nanodroplets formed both at intrinsic defects of the FOTS monolayer and at the FOTS/SiOx interface. Capillary bridges of liquid water are dominantly formed in the confined particle contact areas and between surface asperities on the particles. The comparison of wetting, adsorption and capillary bridge formation shows that the hydrophobization of porous materials by organosilane monolayers leads to the formation of morphology dependent nanoscopic defects that act as sites for preferential capillary bridge formation.

Wettability-enabling coating on oxide particles through controlled milling

The purpose of this paper is to describe a metal deposition process which aims at getting a wettability-enhancing tin coating on silica particles. This original mechano-synthesis protocol allows the deposition of thin (nm) metal layer on oxide, while maintaining particles integrity. Macroscopic wettability tests in liquid tin/bismuth alloy confirm the efficiency of this approach for the production of metal matrix/oxide inclusions composites, while microscopic imaging have shown that individual wetting of particles can easily be attained with the appropriate mixing conditions.

Femtosecond laser-induced superwetting surfaces

The materials with various superwettabilities have attracted increasing interests because of their broad applications, such as water/oil-repellent coating, self-cleaning coating, anti-ice/fog/snow, manipulation of droplets, oil/water separation, antifouling, anti-corrosion, underwater drag reduction, lab chip, cell engineering, fog collection, microfluidic systems, and so on. After millions of years of evolution, natural organisms have evolved perfect structures and multifunctional surfaces. The surfaces of many plants and animals are endowed with special wettability, which inspire scientists to design and fabricate bionic surfaces with superwettability. It is found that the surface wettability is mainly determined by the chemical composition and surface microstructure. Inspired by nature (e.g., lotus leaf, fish scale, pitcher plant, red rose petal, rice leaf, butterfly wings), a large number of superwetting surfaces have been developed by various microfabrication technologies. However, those methods, more or less, are suffered from some inherent limitations, e.g., tight restriction on special materials, lack flexibility, and complex fabrication process. The development of a widely applicable and simple tool that can easily achieve various superwettabilities is still the major trend in this research field. Femtosecond laser has been proven to be a strong tool in advanced micro/nanofabrication. Recently, this tool is also successfully applied to design and change the surface wettability of different solid substrates. The femtosecond laser microfabrication has many advantages, such as small heat-affected zone, precise ablation threshold, non-contact process, and high resolution. In addition, the femtosecond laser can ablate almost all of the materials, including semiconductors, metals, polymers, glasses, and ceramics. By the simple one-step scanning manner, micro/nanoscale hierarchical structures can be directly created on the surfaces of different materials via the femtosecond laser treatment. Taking advantages of these features, femtosecond laser microfabrication have also been a great success in preparing superwetting surfaces. In this review, we summarize the recent developments in femtosecond laser-structured surfaces with special wettability. The article starts with the introduction related to the theoretical basis of wettability and the features of femtosecond laser microfabrication, as the background. Then, different superwettabilities achieved by femtosecond laser treatment are summarized, grouped by the characteristic of wettability. Inspired by nature, those laser-induced surface microstructures have superhydrophobicity, underwater superoleophobicity, underwater superaerophobicity, slippery liquid-infused porous surface, controllable adhesion, and anisotropic wettability, respectively. In each class of properties, the biological surface with such superwettability is introduced in advance. Then, how to use a femtosecond laser to design and fabricate these special superwetting microstructures was discussed, i.e., the preparation principle. According to the formation mechanism of these superwetting states, the internal relation between the femtosecond laser-induced surface microstructure and the macroscopic wettability is revealed; that is, surface microstructure determines the surface wettability. Next, some examples about the practical applications of the femtosecond laser-designed superwetting surfaces are also introduced following various superwettabilities, such as self-cleaning, manipulation of liquid droplets, oil/water separation, cell engineering, fog harvesting, buoyancy enhancement, liquid patterning, underwater gas capture, and detection analysis. Finally, the existing challenges and future prospects of this fast-growing field are discussed. The technology of femtosecond laser-controlled surface wettability is still irreplaceable by other microfabrication methods because of its special advantages. Such technology is better at designing and preparing fine microstructures and complex and heterogeneous wettability. This technology will have superiority in some special applications, such as cell engineering, military facilities, etc. With more and more scientists and engineers getting into this research field and being devoted to fabricating various superwetting surfaces, this research direction will have a bright and exciting future.

Controlling Droplet Motion on an Organogel Surface by Tuning the Chain Length of DNA and Its Biosensing Application

Summary Controllable droplet motion has attracted extensive attention but still faces the challenges of an uncontrollable sliding speed and a slow response rate. Here, controllable sliding speed and critical sliding angle (CSA) of a droplet on an oil-swollen organogel surface are achieved with a fast response rate via modulation of the DNA chain length. Comprehensive investigations ranging from macroscopic wetting behavior to molecular mechanism suggest that short single-stranded DNA (ssDNA) could act as a hydrotrope to interact with oil molecules via interfacial hydrophobic interactions while increasing the interfacial thickness and adhesion, thus hindering the movement of droplets. Conversely, long ssDNA generated from rolling-circle amplification tend toward a curled conformation, minimizing nucleobase exposure and leading to weak interfacial adhesion, allowing the droplets to slide easily. With a significant CSA gradient, biosensing applications for ATP, microRNA, and thrombin detection are demonstrated, indicating the potential for the detection of a broad range of targets.

From Molecular Arrangement to Macroscopic Wetting of Ionic Liquids on the Mica Surface: Effect of Humidity

To optimize the wetting performance of ionic liquids (ILs) on solid surfaces, which is important in catalysis, lubrication, and energy storage, it is critical to control the molecular arrangement of ILs at the IL/solid interface. Here, we report our experimental results, showing that tuning humidity is a facile and effective approach manipulating the molecular arrangement and thus controlling the macroscopic wettability of ILs on the mica surface. Fourier transform infrared spectroscopy, contact angle testing, and atomic force microscopy results showed that with the increase of humidity, more water adsorbs on the mica surface, which dissolves and mobilizes K+ on the mica. As a result, the cations of ILs occupy the empty spot left by the K+ and initiate the layering of ILs. The water-enabled ion exchange and IL layering processes result in not onlythe decrease of the IL contact angle on the mica but also the time-dependent contact angle. The finding here potentially provides a new dimension tailoring the performance of ILs at the IL/solid interface.

Understanding the Wettability of Calcite (CaCO3) Using Higher Spatial Resolution

Wettability alteration of carbonate rocks from an oil-wet state to a water wet or mixed-wet state during water flooding is known to enhance the recovery of oil from the reservoirs. Previously, the wettability of porous mediums has been extensively investigated using numerous macroscopic methods. Among those, the contact angle is a highly employed method for wettability measurements; however, this technique, because of its lower spatial resolution, fails to provide a complete chemical understanding of all the factors affecting the reservoir wettability. In an attempt to overcome this, we employed a multiscale approach involving macro-, micro-, and nanoscopic analytical techniques to investigate the wettability of calcite. Studies were performed by aging two different planes of freshly cleaved calcite in ambient atmosphere and with deionized (DI) water. Contact angle measurements and AFM force profiles were recorded at the macroscale and nanoscale, respectively. Wettability transition was observed from super hydrophilic to hydrophobic nature in ambient atmosphere and super hydrophilic to hydrophilic nature in DI water. When AFM studies were performed on samples aged in DI water there were always patches of water present, which were observed only at the nanoscale. These water patches affect the contact angle measurements and make the macroscopic wettability results inherently ambiguous. This work has shown that the contact angle measurements should not be taken as the absolute measurement of wettability.

Rapid, Template‐Less Patterning of Polymeric Interfaces for Controlled Wettability via in Situ Heterogeneous Photopolymerizations

AbstractTuning macroscopic behavior between a synthetic surface and the surrounding environment requires precise engineering of micro‐ and nanoscale features at the interface. As one example, the geometry and spacing of topographical structures can be manipulated to modify interfacial interactions with water. If optimized correctly, topographical features can impart macroscopic wetting behavior that goes well beyond what is expected from the chemistry of the surface materials, such as strong hydrophobicity from intrinsically hydrophilic materials. However, developed techniques such as templating or nano‐lithography require significant processing times and are often ill‐suited for in situ applications. This work shows that polymeric interfaces with regular, ordered topographical features can be generated using a rapid in situ photopolymerization technique that requires no templating nor additional processing to generate or manipulate surface features. Maximizing the local differential in polymerization‐induced shrinkage between heterogeneous domains results in the formation of robust 3D surface features. Furthermore, the spacing and density of these features can be optimized through the phase separation process, yielding surfaces with variable wetting (θw ≈ 90–130°). These results demonstrate that template‐ and mask‐less surface patterning is possible with rapid polymerization techniques and that it can be easily manipulated to vary surface wetting behavior.

Links between nanoscale and macroscale surface properties of natural root mucilage studied by atomic force microscopy and contact angle

Soil water repellency originating from organic coatings plays a crucial role for soil hydraulics and plant water uptake. Focussing on hydrophobicity in the rhizosphere induced by root-mucilage, this study aims to explore the link between macroscopic wettability and nano-microscopic surface properties. The existing knowledge of the nanostructures of organic soil compounds and its effect on wettability is limited by the lack of a method capable to assess the natural spatial heterogeneity of physical and chemical properties.In this contribution, this task is tackled by a geostatistical approach via variogram analysis of topography and adhesion force data acquired by atomic force microscopy and macroscopic sessile drop measurements on dried films of mucilage. The results are discussed following the wetting models given by Wenzel and Cassie-Baxter.Undiluted mucilage formed homogeneous films on the substrate with contact angles >90°. For diluted samples contact angles were smaller and incomplete mucilage surface coverage with hole-like structures frequently exhibited increased adhesion forces. Break-free distances of force curves indicated enhanced capillary forces due to adsorbed water films at atmospheric RH (35 ± 2%) that promote wettability. Variogram analysis enabled a description of complex surface structures exceeding the capability of comparative visual inspection.

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.

Understanding the wettability of nanometer-thick room temperature ionic liquids (RTILs) on solid surfaces

Many important applications of room temperature ionic liquids (RTILs), e.g., lubrication, energy storage and catalysis, involve RTILs confined to solid surfaces. In order to optimize the performance, it is critical to understand the wettability of nanometer-thick RTILs on solid surfaces. In this review, the recent progress in this filed is presented. First, the macroscopic wettability of RTILs on solids will be discussed briefly. Afterwards, the wetting of nanometer-thick RTILs will be discussed with the emphasis on RTIL/mica and RTIL/graphite interfaces since mica and graphite not only are mostly studied but also have important real-life applications. For RTIL/mica interface, the extended layering that promotes the wetting has been extensively reported and it is generally accepted that the electrostatic interaction at the RTIL/mica interface is the key. However, recent works from others and us highlight the unexpected effect of water: Water enables ion exchange between K+ and the cations of RTILs on the mica surface and thus triggers the ordered packing of cations/anions in RTILs, resulting in extended layering. Different from mica, there is no electrical charge on the graphite surface. Interestingly, previous reports showed inconsistent results on the wettability of RTILs on graphite. Recent research from others and us suggested that π-π+ stacking between sp2 carbon and the imidazoliumcation in the RTILs is the key to the extended layering and enhanced wettability of RTILs. Lastly, the future research directions will be briefly discussed.

Pore‐Scale Distribution of Mucilage Affecting Water Repellency in the Rhizosphere

Core Ideas During drying, mucilage is preferentially deposited in small pores. This microscopic heterogeneity critically affects macroscopic wettability. Infiltration is impeded when a critical fraction of pores is blocked by dry mucilage. Dry mucilage bridges are shaped like hollow cylinders connecting particles. The physical properties of the rhizosphere are strongly influenced by root‐exuded mucilage, and there is increasing evidence that mucilage affects the wettability of soils on drying. We introduce a conceptual model of mucilage deposition during soil drying and its impact on soil wettability. We hypothesized that as soil dries, water menisci recede and draw mucilage toward the contact region between particles. At low mucilage contents (milligrams per gram of soil), mucilage deposits have the shape of thin filaments that are bypassed by infiltrating water. At higher contents, mucilage deposits occupy a large fraction of the pore space and make the rhizosphere hydrophobic. This hypothesis was confirmed by microscope images and contact angle measurements. We measured the initial contact angle of quartz sand (0.5–0.63‐ and 0.125–0.2‐mm diameter), silt (36–63‐μm diameter), and glass beads (0.1–0.2‐mm diameter) mixed with varying amounts of chia (Salvia hispanica L.) seed mucilage (dry content range 0.2–19 mg g−1) using the sessile drop method. We observed a threshold‐like occurrence of water repellency. At low mucilage contents, the water drop infiltrated within 300 ms. Above a critical mucilage content, the soil particle–mucilage mixture turned water repellent. The critical mucilage content decreased with increasing soil particle size. Above this critical content, mucilage deposits have the shape of hollow cylinders that occupy a large fraction of the pore space. Below the critical mucilage content, mucilage deposits have the shape of thin filaments. This study shows how the microscopic heterogeneity of mucilage distribution impacts the macroscopic wettability of mucilage‐embedded soil particles.

Effect of steric hindrance on surface wettability of fine silica powder modified by n- or t-butyl alcohol

Fumed silica is an important industrial material, which is widely used in medical, cosmetic, and electronic products. One important industrial application of the fumed silica powder is using it as fillers, which are key materials for reinforcing the high-performance/high-functional industrial products. However, the use of fumed silica as filler is limited to micrometer or millimeter sized particles, because nanometer sized silica particles tend to aggregate. In this study, the surface of fine silica powder is modified with n- and t-butyl alcohols, which exhibit different steric hindrance effects. The surface wettability of each modified silica powders is determined in two ways: macroscopic and microscopic wettability. Macroscopic wettability refers to preference dispersion test and microscopic wettability refers to evaluation of the molecular level by Fourier transform infrared spectroscopy (FT-IR). The modification ratio of each sample is confirmed by thermogravimetry-differential thermal analysis (TG-DTA), and the hydrophobicity of these modified silica powders is evaluated by the preference dispersion test. Molecular level evaluation of surface wettability by FT-IR confirms an obvious structural difference due to the steric hindrance of the n- and t-butoxy groups on the surface of silica. In addition, a correlation between macroscopic and microscopic evaluation results for the surface wettability of modified silica powders is confirmed.

Superwettability integration: concepts, design and applications

Functional surfaces exhibiting superwettability offer great opportunities to develop advanced science and technology. To unite the advantages of different surfaces with special wettability, superwettability integration has been proposed and fabricated, showing a co-operative and complementary effect on material properties. This paper discusses how to combine different wettabilities and what functions can be achieved based on superwettability integration. Superwettability integration is defined as a unified system combining different superwetting materials or assembling superwetting materials with other functional materials. Four types of superwettability integrations are summarized: (a) surfaces with microscopic wettability patterns, (b) surfaces with macroscopic wettability patterns, (c) Janus interface materials with asymmetric wettability and (d) surfaces with specific wettability boundary. The functions and applications of the integrated systems are reviewed, particularly for use in fluid control, vapor condensation, fog collection and so on. The principle of designing the integration is also depicted as manipulating the significant difference in surface energy between two surfaces. On the basis of current research, some of the possible designs are finally proposed and a prospect of multifunctional superwettability integration is made.

Experimental and simulation study of carbon dioxide, brine, and muscovite surface interactions

Capture and subsequent geologic storage of CO2 in deep brine reservoirs plays a significant role in plans to reduce atmospheric carbon emission and resulting global climate change. Subsurface injection of CO2 is also used industrially in enhanced oil and natural gas recovery operations to increase the amount of hydrocarbon that can be economically recovered from a geologic reservoir. The interaction of CO2 and brine species with mineral surfaces controls the ultimate fate of injected CO2 at the nanoscale via surface chemistry, at the pore-scale via capillary trapping, and at the field-scale via relative permeability. High resolution micro X-ray CT scanning, optical contact angle measurements, and large scale molecular dynamics simulations were used to investigate the behavior of supercritical CO2 and aqueous fluids on basal surfaces of muscovite, a common phyllosilicate mineral. Experimental results demonstrate partial wetting by the aqueous phase and a dependence of contact angle upon aqueous phase brine composition. This contrasts with simulation results, which predict that supercritical CO2 forms a non-wetting droplet, separated from direct interaction with the muscovite surface by distinct layers of water and charged species. Simulations with trace amounts of acetate or acetic acid added to the CO2/water/mineral system were used to investigate the potential effect of contamination with small organic molecules. While the observed contact angle was not significantly altered, these simulations demonstrate the influence of pH on species partitioning, with acetic acid molecules partitioning to the CO2/water interface and acetate ions adsorbing to the mineral surface. Similar simulations using hexanoate displayed a greater surfactant effect and significantly increased wetting by the CO2 phase, suggesting that small concentrations of secondary species or contaminants can significantly influence macroscopic wetting behavior.

Influence of Water on the Interfacial Nanostructure and Wetting of [Rmim][NTf2] Ionic Liquids at Mica Surfaces.

The effect of water concentration on the interfacial nanostructure and wetting behavior of a family of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2], at the surface of mica was investigated by contact angle measurement and atomic force microscopy (AFM). AFM reveals that interfacial layers of ILs observed at the surface of mica for "dry" ILs are not present for water-saturated ILs. The interaction of the IL ions of [Rmim][NTf2] with water molecules through hydrogen bonding is suspected to disrupt IL ion layering and precursor film growth on mica. Without the IL precursor film, contact angle relaxation of "wet" ILs on mica is less significant and ambient vapor adsorption becomes more important in determining the macroscopic wetting behavior.

AFM Study of Surface Nanobubbles on Binary Self-Assembled Monolayers on Ultraflat Gold with Identical Macroscopic Static Water Contact Angles and Different Terminal Functional Groups.

All experimental findings related to surface nanobubbles, such as their pronounced stability and the striking differences of macroscopic and apparent nanoscopic contact angles, need to be addressed in any theory or model of surface nanobubbles. In this work we critically test a recent explanation of surface nanobubble stability and their consequences and contrast this with previously proposed models. In particular, we elucidated the effect of surface chemical composition of well-controlled solid-aqueous interfaces of identical roughness and defect density on the apparent nanoscopic contact angles. Expanding on a previous atomic force microscopy (AFM) study on the systematic variation of the macroscopic wettability using binary self-assembled monolayers (SAMs) on ultraflat template stripped gold (TSG), we assessed here the effect of different surface chemical composition for macroscopically identical static water contact angles. SAMs on TSG with a constant macroscopic water contact angle of 81 ± 2° were obtained by coadsorption of a methyl-terminated thiol and a second thiol with different terminal functional groups, including hydroxy, amino, and carboxylic acid groups. In addition, surface nanobubbles formed by entrainment of air on SAMs of a bromoisobutyrate-terminated thiol were analyzed by AFM. Despite the widely differing surface potentials and different functionality, such as hydrogen bond acceptor or donor, and different dipole moments and polarizability, the nanoscopic contact angles (measured through the condensed phase and corrected for AFM tip broadening effects) were found to be 145 ± 10° for all surfaces. Hence, different chemical functionalities at identical macroscopic static water contact angle do not noticeably influence the apparent nanoscopic contact angle of surface nanobubbles. This universal contact angle is in agreement with recent models that rely on contact line pinning and the equilibrium of gas outflux due to the Laplace pressure and gas influx due to gas oversaturation in the aqueous medium.

Complete wetting of graphene by biological lipids.

Graphene nanosheets have been demonstrated to extract large amounts of lipid molecules directly out of the cell membrane of bacteria and thus cause serious damage to the cell's integrity. This interesting phenomenon, however, is so far not well understood theoretically. Here through extensive molecular dynamics simulations and theoretical analyses, we show that this phenomenon can be categorized as a complete wetting of graphene by membrane lipids in water. A wetting-based theory was utilized to associate the free energy change during the microscopic extraction of a lipid with the spreading parameter for the macroscopic wetting. With a customized thermodynamic cycle for detailed energetics, we show that the dispersive adhesion between graphene and lipids plays a dominant role during this extraction as manifested by the curved graphene. Our simulation results suggest that biological lipids can completely wet the concave, flat or even convex (with a small curvature) surface of a graphene sheet.

Wettability Alteration during Low-Salinity Waterflooding and the Relevance of Divalent Ions in This Process

From laboratory and field test results, it is now largely agreed that reducing the overall salinity, especially the concentration of divalent ions of the injected water, can markedly improve the oil recovery. However, as a result of the complexity of the crude oil–brine–rock (COBR) system, there is no clear explanation of why the divalent cations (Ca2+) are of major significance in a low-salinity water effect (LSWE). In the present paper, spontaneous imbibition, ζ-potential measurements, static adsorption/desorption of benzoic acid (BA) onto crushed Berea, and BA self-assembled layers on silica wafer have been performed to correlate macroscopic wettability alteration during low-saline water flood with the microscopic release of the hydrophobic layers (organic acid layers) on mineral surfaces and explain the relevance of active cations (Ca2+) in this process, thereby providing a better understanding of the underlying mechanisms of a LSWE. Spontaneous imbibition results show that initial wettability of the ...

Hydrodynamic confinement and capillary alignment of gold nanorods

Controlling the alignment and orientation of nanorods on various surfaces poses major challenges. In this work, we investigate hydrodynamic confinement and capillary alignment of gold nanorod assembly on chemically stripe-patterned substrates. The surface patterns consist of alternating hydrophilic and hydrophobic micrometer wide stripes; a macroscopic wettability gradient enables controlling the dynamics of deposited suspension droplets. We show that drying of residual liquid on the hydrophilic stripes gives rise to spatially localized deposition and alignment of the nanorods. Moreover, a universal relation between the extent of order within the single layers of nanoparticles and the lateral dimension of the deposits is presented and discussed.