Helium wetting: Theme and variations on inhomogeneous helium

The surprising discovery, in 1991, that liquid helium does not wet a cesium surface at low temperature has triggered an important activity both theoretical and experimental: helium has become a model system for the study of wetting transitions. After summarizing the main theme of helium wetting, I will focus on more recent studies, such as the structure and excitations of helium interfaces, experiments on the capillary rise, the “surfactant effect” of helium-3 impurities on liquid helium-4 and the “quantum prewetting transition” of pure helium-3. Unexpected consequences on the phase separation of³He?⁴He mixtures in restricted geometry will be drawn.     

Polytherms of angles of wetting by tin-strontium melts for aluminum films on silicon before and after photon annealing

Polytherms of the angles of wetting by Sn-Sr melts for aluminum films on silicon are investigated by the sessile drop method in helium. Such melts are promising for use as lead-free solders in electronics. It is found that the photon annealing of films enhances their wetting. Wetting thresholds are found at temperatures of >850 K for tin-strontium melts spreading over the surfaces of aluminum films on silicon that are preliminarily processed by photon annealing for 4 s.     

Negative Ca− and Ba− ions of large radius on the surface and in the volume of liquid helium

Negative Ca⁻ and Ba⁻ ions of large radii on the surface of and in bulk liquid helium have been studied. Our results indicate that these ions are adsorbed on the helium surface. Ions on free liquid helium surfaces have not been studied previously because it was thought impossible to confine them on the surface. Ca⁻ and Ba⁻ ions have very low binding energies, therefore, like electrons, they form a bubble of large radius in bulk helium, whose energy is higher than on the surface. The behavior of ions on the surface exhibits a number of previously unknown features owing to their large masses and strong localization in the horizontal plane. Even in the absence of confining electric field, a hole is formed under an ion due to the polarization attraction between the liquid helium and the charged ion. This hole formation reduces the ion mobility by several orders of magnitude and increases its effective mass severalfold. The critical density of electrons and ions is approximately the same on the surfaces of thin and thick helium films. Zh. éksp. Teor. Fiz. 115, 593–604 (February 1999)     

Theory of the resonant properties of electrons localized on the surface of liquid helium

The problem of the shape of the line of optical transition of an electron between bound states on the surface of liquid helium is solved within the independent boson model. Such bound states are realized, for example, in the potential of a positively charged impurity located on a substrate or in the field of a He⁺ ion located beneath the surface. Reference is made to the importance of the relaxation processes of the dimple on the helium surface under the electron. The adiabatic approximation, in the case of which the dimple does not change during the time of electron transition, is not always valid. At low temperatures, two maxima may appear on the absorption line. It is demonstrated that the far tails of the optical absorption line feature a universal (Urbach rule) exponential dependence on the electron transition energy.     

The density of helium in bubbles in implanted materials: Results from VUV absorption and eel spectroscopy

Abstract

The novel application of vacuum ultra-violet absorption spectroscopy and electron energy loss spectroscopy to helium bubbles in metals is presented. These measurements, carried out on thin aluminium films containing different concentrations of helium and various bubble size distributions, were aimed at determining the density (and thus pressure) of helium in bubbles by observing the shift and broadening of the IS-2P transition in the helium. The data coupled with a theoretical model developed by the authors (see following paper) indicate densities as high as 10²³ He cm^?3 for specimens containing small bubbles. Data are also presented on the effect that annealing and cooling have on these spectra. The annealing experiments give rise to fairly complex changes in absorption peak structure but with a general shift towards the unperturbed resonance line. The cooling experiment gives rise to a further shift and a narrowing of the absorption spectrum on cooling to 77 K which is tentatively identified as the liquid/solid transition in the helium. Finally, fluorescence spectrum of an Al/He specimen excited with low energy electrons is presented.     

Zur Supraleitung von Doppelschichten aus Ag/Sn

Superimposed films of Ag and Sn are produced on suitable conditions of condensation, so that diffusion between the layers is prevented (the condensation temperatures of Ag and Sn are 320 ?K and 150 ?K, respectively). Immediately after condensation the films are cooled to liquid helium temperature, and the transition temperatures of superconductivity are measured. These values are in agreement with a phenomenological theory of P. and R.Hilsch for sufficient thick films. Resistance measurements indicate that an alloy of Ag and Sn is formed above 280 ?K. There is only a small shift of transition temperature caused by the alloying process.     

Experimental study on fuel drop impacts onto rigid surfaces: Morphological comparisons, disintegration limits and secondary atomization

This paper addresses an experimental study of the effects of the topography of surfaces impacted by fuel sprays on the process of fuel–air mixing in internal combustion engines. The experiments reported here consider the use of a simplified flow configuration consisting of individual droplets impacting onto flat surfaces with different surface topographies, roughness and temperatures. The main parameters which were systematically varied are surface wettability and topography and liquid viscosity. The analysis considers the onset of splash, which involves the identification and characterization of different disintegration mechanisms. The characterization is performed in terms of crown morphology, temporal evolution and secondary droplet characteristics, for different boiling regimes. The results are interpreted to better understand the influence of the nature of the surface in the efficiency of the disintegration mechanisms.     

Mechanochemical penetration of helium atoms into Pd84.5-Si15.5 and Ni78-Si8-B14 eutectic amorphous films extended in liquid 3He and 4He

The penetration of helium atoms into amorphous films extended to fracture in liquid helium has been investigated. It is found that helium atoms penetrate into the eutectic alloy films Pd_84.5-Si_15.5 in ³He (T=0.5 K) and Ni₇₈-Si₈-B₁₄ in ⁴He (T=4.2 K). The spectra of helium liberation from these materials after deformation are obtained upon dynamic (4–5 K/min) annealing at T=293–1323 K. The maximum amount of helium is observed in the regions of local plastic microshears running across the whole width of films and also in the sample regions containing fracture macrocracks and isolated groups of slip bands. The spectra of helium liberation from different regions of destroyed samples show several peaks that correlate with the temperatures of crystallization and melting of the studied films. The data obtained are interpreted within the model of mechanochemical penetration of helium atoms through the dynamically excited dislocation-like defects, which are typical of the amorphous films under consideration.     

Adsorption and superfluid onset of He films on intermediate strength substrates

On strong binding substrates, such as graphite or mylar that are wetted by ⁴He at all temperatures, an adsorbed ⁴He film consists of 2 atomic layers of “inert” helium covered by a liquid layer that becomes superfluid via a Kosterlitz–Thouless (KT) transition. On weak substrates, for example cesium, superfluid onset above the wetting temperature also conforms to the KT picture. In contrast, superfluid onset on intermediate strength substrates, specifically heavier alkali metals and monolyer films of cesium on gold, deviates strongly from KT behavior. Here we describe superfluid onset of ⁴He on intermediate strength substrates and discuss the contributions of weak bindings and disorder to the non-KT behavior.     

How the thickness of a liquid helium film influences the spectrum of electrons localized above its surface

We consider the problem of finding the energy spectrum of electrons localized above the surface of a liquid helium film under the electrostatic attraction of induced charges. We show that the forces of attraction to the charges induced on the surface of a conductor located beneath the film begin to additionally influence the forces of attraction of the electrons to the free liquid surface as the film thickness decreases. When the film thickness becomes less than 10μm, this influence causes a significant increase in the energy difference between the lower levels and a decrease in their depth. We suggest a numerical method for solving the problem and present the results of our calculations of the energy spectrum of electrons localized above the surfaces of liquid ³He, ⁴He, and neon films. The influence of the pressing electric field on the energy spectrum is considered.     

Image forces and electron spectrum at the surface of liquid helium

On the basis of the longitudinal field Green function, in a three-layer non-symmetrical system of media with spatial dispersion the image-force potential is calculated for surfaces of the semi-infinite helium bath and a liquid helium film, and also for a surface of the phase-separated ³He⁴He mixture. The dependence of the surface electron spectrum on the film thickness and on the pressing electric field strength is analyzed.     

正癸烷纳米液滴润湿特性的分子动力学研究

流体液滴在固体表面的浸润性对其润滑性能至关重要.本文利用分子动力学方法研究了正癸烷纳米液滴在铜表面上的润湿特性.结果表明：在平坦光滑表面上,壁面的厚度和分子数目对润湿效果影响不大.随着壁面能量势阱参数εs 增大,接触角线性减小.随着温度升高,液滴的接触角减小.在沟槽粗糙表面上,随着粗糙度因子增大,对于疏液表面,接触角增大到一定值后基本保持不变,符合Cassie理论;中性和亲液表面接触角则会减小,为Wenzel润湿模式.当表面分数增大时,疏液与亲液表面接触角整体呈减小的趋势,对中性表面影响不大.当温度升高时,粗糙疏液表面接触角会增大,润湿效果更差,而粗糙中性和亲液表面液滴润湿性会更好.     

Hydrostatic-Pressure-Induced Reentrance of the Metallic State in the κ-(ET)2Hg(SCN)2Cl Quasi-Two-Dimensional Organic Conductor

On cooling below 30 K, the κ-(ET)₂Hg(SCN)₂Cl quasi-two-dimensional organic metal, which is in the quantum spin liquid state at liquid helium temperatures, undergoes a transition to the Mott insulator state. The application of a hydrostatic pressure p = 0.7 kbar stabilizes the metallic state and makes it possible to study the behavior of the interlayer magnetoresistance at liquid helium temperatures. The field dependence of the magnetoresistance exhibits an unlimited power-law growth, which indicates that the polaron mechanism contributes to the interlayer transport. The spectrum of observed magnetoresistance oscillations corresponds to the Fermi surfaces characteristic of conducting layers with the κ-type structure.

     

The apparent state of droplets on a rough surface

The factors influencing the state and wetting transition of droplets on a rough surface are both complex and obscure. The change in wetting is directly reflected by changes under the contact condition of the droplets with the surface. The recent study about the wettability of the superhydrophobic surface under the condensing condition arouses the new understanding about the apparent state of droplets on a rough surface. In this work, to validate the existence of droplets in an intermediate state, a microscale pillar topological polydimethylsiloxane (PDMS) surface was manufactured and its wettability under various conditions was studied. According to the experimental data, it is proposed that the wetting state of a rough surface may be embodied using the contact area ratio of a solid/liquid/gas droplet with the projective plane. A general calculation model for the apparent contact angle of droplets is given and expressed diagrammatically. It is found that the measured apparent contact angles of droplets at different states on the surface falls within the range predicted by our proposed equation. Supported by the National Natural Science Foundation of China (Grant No. 50606025)     

Topography driven spreading

Roughening a hydrophobic surface enhances its nonwetting properties into superhydrophobicity. For liquids other than water, roughness can induce a complete rollup of a droplet. However, topographic effects can also enhance partial wetting by a given liquid into complete wetting to create superwetting. In this work, a model system of spreading droplets of a nonvolatile liquid on surfaces having lithographically produced pillars is used to show that superwetting also modifies the dynamics of spreading. The edge speed-dynamic contact angle relation is shown to obey a simple power law, and such power laws are shown to apply to naturally occurring surfaces.     

The multilayer melting transition in methane adsorbed on graphite

High resolution heat capacity measurements of multilayer methane adsorbed on graphite are presented and analyzed. The evidence indicates the presence of two wetting transitions: a first-order dewetting transition at T_w = 90.48 K, and a continuous wetting transition at the triple point, T_t = 90.66 K. This behavior is to be expected in connection with the melting transition in any system where both solid and liquid wet the surface. Heat capacity measurements can provide a valuable diagnostic tool for the wetting behavior of films too thick to be investigated by other means. In the thin film limit, we find that the latent heat of melting vanishes at about 4 layers.     

Probing helium interfaces with light scattering: From fluid mechanics to statistical physics

We have investigated the formation of helium droplets in two physical situations. In the first one, droplets are atomised from superfluid or normal liquid by a fast helium vapour flow. In the second, droplets of normal liquid are formed inside porous glasses during the process of helium condensation. The context, aims, and results of these experiments are reviewed, with focus on the specificity of light scattering by helium. In particular, we discuss how, for different reasons, the closeness to unity of the index of refraction of helium allows in both cases to minimise the problem of multiple scattering and obtain results which it would not be possible to get using other fluids.     

Wetting mode transition of nanoliter scale water droplets during evaporation on superhydrophobic surfaces with random roughness structure

During evaporation, shape changes of nanoliter-scale (80-100 nL) water droplets were evaluated on two superhydrophobic surfaces with different random roughness (nm-coating, μm-coating). The square of the contact radius and the square of the droplet height decreased linearly with evaporation time. However, trend changes were observed at around 170 s (nm-coating) and around 150 s (μm-coating) suggesting a wetting mode transition. The calculated droplet radii for the wetting mode transition from the average roughness distance and the average roughness height of these surface structures were approximately equal to the experimental values at these trend changes. A certain level of correlation between the roughness size and droplet radius at the wetting mode transition was confirmed on surfaces with random roughness.     

Two-Dimensional Wetting Transition Modeling with the Potts Model

A droplet of a liquid deposited on a surface structured in pillars may have two states of wetting: (1) Cassie-Baxter (CB), the liquid remains on top of the pillars, also known as heterogeneous wetting, or (2) Wenzel, the liquid fills completely the cavities of the surface, also known as homogeneous wetting. Studies show that between these two states, there is an energy barrier that, when overcome, results in the transition of states. The transition can be achieved by changes in geometry parameters of the surface, by vibrations of the surface or by evaporation of the liquid. In this paper, we present a comparison of two-dimensional simulations of the Cassie-Wenzel transition on pillar-structured surfaces using the cellular Potts model (CPM) with studies performed by Shahraz et al. In our work, we determine a transition diagram by varying the surface parameters such as the interpillar distance (G) and the pillar height (H). Our results were compared to those obtained by Shahraz et al. obtaining good agreement.