Thin Liquid Metal Layer Research Articles

Fury: an experimental dynamo with anisotropic electrical conductivity

We report measurements of dynamo action in a new experimental set-up, named Fury, based on the use of an anisotropic electrical conductivity. It consists of a copper rotor rotating inside a copper stator, electrically connected with a thin layer of liquid metal, galinstan. Grooves have been cut in the copper so that, everywhere, electrical conductivity can be considered to be that of copper along two directions while it is zero along the third one. The configuration is efficient and dynamo action can be powered by hand. We have also used a motor with better control, enabling us to drive the rotor at specified velocity or torque functions of time. The structure of the axisymmetric magnetic field produced is found to be close to the numerical modelling using FreeFem++. The experimental dynamo behaves very nearly as expected for a kinematic dynamo, so that the threshold dynamo velocity cannot be exceeded, or only briefly. More mechanical power in the rotor rotation leads to an increase in the magnetic field intensity, the magnetic energy being proportional to the extra mechanical power beyond the threshold. In the transient following a step increase of torque, magnetic and angular velocity oscillations have been observed and explained.

Stretchable Gas Barrier Films Using Liquid Metal toward a Highly Deformable Battery.

Highly deformable batteries that are flexible and stretchable are important for the next-generation wearable devices. Several studies have focused on the stable operation and life span of batteries. On the other hand, there has been less focus on the packaging of highly deformable batteries. In wearable devices, solid-state or pouch lithium-ion batteries (LIBs) packaged in aluminum (Al)-laminated films, which protect against moisture and gas permeation, are used. Stretchable elastomer materials are used as the packaging films of highly deformable batteries; however, they are extremely permeable to gas and moisture. Therefore, a packaging film that provides high deformability along with gas and moisture barrier functionalities is required for the stable operation of highly deformable batteries used in ambient conditions. In this study, a stretchable packaging film with high gas barrier functionality is developed successfully by coating a thin layer of liquid metal onto a gold (Au)-deposited thermoplastic polyurethane film using the layer-by-layer method. The film exhibits excellent oxygen gas impermeability under mechanical strain and extremely low moisture permeability. It shows high impermeability along with high mechanical robustness. Using the proposed stretchable gas barrier film, a highly deformable LIB is assembled, which offers reliable operation in air. The operation of the highly deformable battery is analyzed by powering LEDs under mechanical deformations in ambient conditions. The proposed stretchable packaging film can potentially be used for the development of packaging films in advanced wearable electronic devices.

Ultrasonic cavitation at liquid/solid interface in a thin Ga–In liquid layer with free surface

Cavitation in thin layer of liquid metal has potential applications in chemical reaction, soldering, extraction, and therapeutic equipment. In this work, the cavitation characteristics and acoustic pressure of a thin liquid Ga–In alloy were studied by high speed photography, numerical simulation, and bubble dynamics calculation. A self-made ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and a max output power of 1000 W during the cavitation recording experiment. The pressure field characteristic inside the thin liquid layer and its influence on the intensity, types, dimensions, and life cycles of cavitation bubbles and on the cavitation evolution process against experimental parameters were systematically studied. The results showed that acoustic pressure inside the thin liquid layer presented alternating positive and negative characteristics within 1 acoustic period (T). Cavitation bubbles nucleated and grew during the negative-pressure stage and shrank and collapsed during the positive-pressure stage. A high bubble growth speed of 16.8 m/s was obtained and evidenced by bubble dynamics calculation. The maximum absolute pressure was obtained at the bottom of the thin liquid layer and resulted in the strongest cavitation. Cavitation was divided into violent and weak stages. The violent cavitation stage lasted several hundreds of acoustic periods and had higher bubble intensity than the weak cavitation stage. Cavitation cloud preferentially appeared during the violent cavitation stage and had a life of several acoustic periods. Tiny cavitation bubbles with life cycles shorter than 1 T dominated the cavitation field. High cavitation intensities were observed at high ultrasonication power and when Q235B alloy was used because such conditions lead to high amplitudes on the substrate and further high acoustic pressure inside the liquid.

Is There a Relationship between Surface Wettability of Structured Surfaces and Lyophobicity toward Liquid Metals?

The liquid metal lyophobicity of a rough substrate was, in previous articles, found to be rather independent on the surface wettability. In this article, we scrutinize the impact of surface wettability of a structured (rough) surface on the liquid metal wettability and adhesion. As a model system, a structured diamond coating was synthesized and modified by air plasma. We show that surface wettability (surface free energy) does not play a prominent role for static contact angle measurements and for the liquid metal repelling properties of the diamond coating in droplet impact experiments. In contrast, roll off angles and repeated deposition experiments illustrate that the increased hydrophilicity impacts the long-term liquid metal repellency of our coating. Liquid metal adhered after around 50 deposition/removal cycles on the hydrophilic diamond coating, while no liquid metal adhesion was visible after 100 cycles on the hydrophobic diamond coating, illustrating the fundamental role for the adhesion of liquid metal. The effect of repeated deposition in conjunction with gentle applied force was employed for coating the liquid metal lyophobic (hydrophilic) diamond coating with a thin liquid metal layer. The observed effect may find application in flexible electronics and thermal management systems as a means to improve interfacing of the liquid metal with conductive non-metal coatings.

A comparative study of the effects of liquid lithium and tin as DEMO divertor targets on the heat loads and SOL properties

The behaviour of the scrape-off plasma of the European tokamak DEMO (DEMOnstration tokamak of the economical feasibility of the fusion power) is analysed by means of the 2D edge code TECXY when either lithium or tin are used as liquid target materials in the single null configuration. The targets are modelled as a thin liquid metal layer superimposed on a tungsten substrate that faces the plasma, while its bottom is kept at a fixed temperature. The top target temperature is calculated from the heat transport equation. The evaporation rate so derived and sputtering give the total impurity source strength. The impurity concentration and the involved radiative losses are calculated self-consistently by solving the multifluid plasma transport equations. A scan in the upstream separatrix density range ne,sep = 2.5–4.5 × 1019 m−3 is carried out for the nominal power input into the scrape off layer, PSOL = 150 MW and in the range PSOL = 150–250 MW for ne,sep ≈ 3.5 × 1019 m−3. Steady state solutions are found only if the maximum plate temperature is kept below the threshold for a very steep increase in evaporation. The radiative capability and peak load mitigation are higher for Sn than for Li but only slightly at high density. If a ceiling to the Sn concentration is set for high core fusion performance, Sn mitigation at low density may drop below Li. Nonetheless, concerns on the Li option are derived from the very large material consumption and on the fact that the main mechanism of impurity release is sputtering, which is outside a full control of the operator. For both Sn and Li, detachment starts to appear only close to the upper limit of the spanned density range. A regime where evaporation overcomes sputtering is more effective in dissipating the input power, especially for Sn. In this case, a sort of vapour shielding seems to develop attached to the impurity source.

INFLUENCE OF CONSTANT MAGNETIC FIELD ON STRUCTURE FORMATION IN STEELS AT HIGH-SPEED LASER PROCESSING

Experimental studies of laser-irradiated layers in a magnetic field (MF) have shown a non-trivial morphology of the surface of handling zone of material in case of reflow. Twisting of a thin layer of liquid metal is observed, irradiated area is getting a crescent appearance, definitely strictly oriented in relation to magnetic flux. This is probably due to the effect of Righi-Leduc, as well as the action of Lorentz forces, which deflect the electrons flow. As a result, there is significant mixing of metal in the irradiation zone, chemical composition equalization, which positively affects the strength properties of the products. One of the important consequences of the MF-effect on the results of laser processing is the phenomenon of magnetostriction. In laser irradiation without MF slide lines were observed on the pre-polished surface patterns resulting from the emerging thermal and structural stresses. By analyzing the topography of irradiated surfaces using modern analysis techniques and computer image processing, it was established that irradiation in MF in conditions of magnetostriction decreases the stress level in irradiated areas and reduces the risk of cracking. The results of temperature measurements at the irradiated spot on cooling stage allow establishing that the cooling rate during laser processing in a MF is considerably higher than without the field. It affects the processes of phase and structural transformations. At laser heating in MF microheterogenic austenite is supercooled with great speed to temperatures of martensite transformation. After that its transformation begins, the sequence of which is determined by the level of local saturation, degree of deformation and is controlled by temperature. The first crystals of martensite are formed in the least saturated areas of austenite, and a very high speed (thousands or tens of thousands of °С/s) of the transformation process beginning γ → α prevents martensite self tempering, which partially can occur when the temperature decreases further due to transformation spread on the remaining volume of austenite, grabbing areas of different saturation. As a result, along with the “fresh-formed” martensite in the areas of laser quenching the martensite is formed, in which segregation of carbon or even ε-carbide may occur and residual austenite with high carbon intensity are formed. Released dispersed carbides contribute to obtaining a sufficiently high hardness values of metals irradiated in a MF.

Nested dipolar vortices driven by electromagnetic forces in a thin liquid metal layer

Nested dipolar vortices driven by electromagnetic forces in a thin liquid metal layer

A superconducting homopolar motor and generator—new approaches

Homopolar machines were the first continuously running electromechanical converters ever demonstrated but engineering challenges and the rapid development of AC technology prevented wider commercialisation. Recent developments in superconducting, cryogenic and sliding contact technology together with new areas of application have led to a renewed interest in homopolar machines. Some of the advantages of these machines are ripple free constant torque, pure DC operation, high power-to-weight ratio and that rotating magnets or coils are not required. In this paper we present our unique approach to high power and high torque homopolar electromagnetic turbines using specially designed high field superconducting magnets and liquid metal current collectors. The unique arrangement of the superconducting coils delivers a high static drive field as well as effective shielding for the field critical sliding contacts. The novel use of additional shielding coils reduces weight and stray field of the system. Liquid metal current collectors deliver a low resistance, stable and low maintenance sliding contact by using a thin liquid metal layer that fills a circular channel formed by the moving edge of a rotor and surrounded by a conforming stationary channel of the stator. Both technologies are critical to constructing high performance machines. Homopolar machines are pure DC devices that utilise only DC electric and magnetic fields and have no AC losses in the coils or the supporting structure. Guina Energy Technologies has developed, built and tested different motor and generator concepts over the last few years and has combined its experience to develop a new generation of homopolar electromagnetic turbines. This paper summarises the development process, general design parameters and first test results of our high temperature superconducting test motor.

A study of gas bubbles in liquid mercury in a vertical Hele-Shaw cell

High-quality observations of mesoscopic gas bubbles in liquid metal are vital for a further development of pyrometallurgical gas injection reactors. However, the opacity of metals enforces the use of indirect imaging techniques with limited temporal or spatial resolution. In addition, accurate interface tracking requires tomography which further complicates the design of a high-temperature experimental setup. In this paper, an alternative approach is suggested that circumvents these two main restrictions. By injecting gas in a thin layer of liquid metal entrapped between two flat and closely spaced plates, bubbles in a Hele-Shaw flow regime are generated. The resulting quasi-2D multiphase flow phenomena can be fully captured from a single point of view and, when using a non-wetted transparent plate material, the bubbles can be observed directly. The feasibility of this approach is demonstrated by observations on buoyancy-driven nitrogen bubbles in liquid mercury in a vertical Hele-Shaw cell. By using a moving high-speed camera to make continuous close up recordings of individual bubbles, the position and geometry of these bubbles are quantified with a high resolution along their entire path. After a thorough evaluation of the experimental accuracy, this information is used for a detailed analysis of the bubble expansion along the path. While the observed bubble growth is mainly caused by the hydrostatic pressure gradient, a careful assessment of the volume variations for smaller bubbles shows that an accurate bubble description should account for significant dynamic pressure variations that seem to be largely regime dependent.

Analytical modelling of a thin liquid metal layer submitted to an ac magnetic field

A cylindrical thin liquid metal layer is submitted to a uniform ac magnetic field. When the intensity of the electromagnetic field exceeds a critical value, an opening in the liquid is shaped from outside to inside. At a given intensity of the electromagnetic field, this opening is in a frozen state, that is, the liquid metal layer reaches a new equilibrium shape. In this paper, we show that this equilibrium corresponds to a minimum of the total energy of the system. This total energy is equal to the sum of the magnetic energy and the mechanical energy. The magnetic energy is computed by assuming that the induced eddy current flowing through the liquid metal layer is concentrated in the cross-section Sc equal to the product of the skin depth and the thickness of the layer. This assumption leads us to study an equivalent electrical circuit.The mechanical energy is composed of the potential energy and the surface energy.

Edge pinch instability of liquid metal sheet in a transverse high-frequency ac magnetic field

We analyze the linear stability of the edge of a thin liquid metal layer subject to a transverse high-frequency ac magnetic field. The layer is treated as a perfectly conducting liquid sheet that allows us to solve the problem analytically for both a semi-infinite geometry with a straight edge and a thin disk of finite radius. It is shown that the long-wave perturbations of a straight edge are monotonically unstable when the wave number exceeds the critical value k(c) = F0/(gamma l0), which is determined by the linear density of the electromagnetic force F0 acting on the edge, the surface tension gamma, and the effective arclength of edge thickness l0. Perturbations with wavelength shorter than critical are stabilized by the surface tension, whereas the growth rate of long-wave perturbations reduces as similar to k for k --> 0. Thus, there is the fastest growing perturbation with the wave number k max = 2/3 k(c). When the layer is arranged vertically, long-wave perturbations are stabilized by the gravity, and the critical perturbation is characterized by the capillary wave number k(c) = square root of (g rho/gamma), where g is the acceleration due to gravity and rho is the density of metal. In this case, the critical linear density of electromagnetic force is F(0,c) = 2k(c)l0 gamma, which corresponds to the critical current amplitude I(0,c) = 4 square root of (pi k(c) l0L gamma/mu 0) when the magnetic field is generated by a straight wire at the distance L directly above the edge. By applying the general approach developed for the semi-infinite sheet, we find that a circular disk of radius R0 placed in a transverse uniform high-frequency ac magnetic field with the induction amplitude B0 becomes linearly unstable with respect to exponentially growing perturbation with the azimuthal wave number m = 2 when the magnetic Bond number exceeds Bm(c) = B(0)2 R(0)2 / (2 mu 0 l0 gamma) = 3 pi. For Bm > Bm(c), the wave number of the fastest growing perturbation is m(max) = [2Bm/(3 pi)]. These theoretical results agree well with the experimental observations.

A computation and a use of the magnetic energy in a thin liquid metal submitted to an AC magnetic field

A thin layer of liquid metal is submitted to a time-dependent, but uniform, magnetic field. The liquid gets an equilibrium shape that depends on the magnetic field intensity. A numeric model, that allows us to explain this behavior, is emphasized

Determination of stable shapes of a thin liquid metal layer using a boundary integral method

This paper deals with a thin liquid metal layer submitted to an ac magnetic field. Experimentally, we have noticed that even if the system (inductor+liquid metal) is axisymmetric, when an ac magnetic field is applied the symmetry is broken. The observed deformations of the liquid metal are in three dimensions. Therefore, our aim is to investigate this deformation using a numerical method as boundary element method in three dimensions.

Neutral satellites in ion beams from liquid metal sources

A strong electric field acting on the tip of a needle anode covered with a thin layer of liquid metal draws out a cusp from which a beam of positive ions is emitted. However, the rate of loss of ionic mass was found to be only a fraction of the total mass loss while the larger part consisted of microparticles of size less than 1 mu m with a low specific positive surface charge. The presence of a neutral component was proven by examining the particles that are adjuncts to the decaying ion beam. It transpired that the main beam constituents are accompanied by relatively few fast neutral atomic 'satellites' with energies spread over a very wide range.

Electromechanical transients in liquid metal on field disruption

The present paper deals with the transient electromechanical interaction in a thin layer of liquid metal on sudden disruption of an imposed magnetic field. The problem is of fundamental importance for understanding the transient performance of various magnetohydrodynamic devices, such as electromagnetic agitators, pumps, liquid metal machines, etc.