Surface tension plays a core role in dominating various surface and interface phenomena. For liquid metals with high melting temperature, a profound understanding of the behaviors of surface tension is crucial in industrial processes such as casting, welding, and solidification, etc. Recently, the room temperature liquid metal (RTLM) mainly composed of gallium-based alloys has caused widespread concerns due to its increasingly realized unique virtues. The surface properties of such materials are rather vital in nearly all applications involved from chip cooling, thermal energy harvesting, hydrogen generation, shape changeable soft machines, printed electronics to 3D fabrication, etc. owing to its pretty large surface tension of approximately 700 mN/m. In order to promote the research of surface tension of RTLM, this paper is dedicated to present an overview on the roles and mechanisms of surface tension of liquid metal and summarize the latest progresses on the understanding of the basic knowledge, theories, influencing factors and experimental measurement methods clarified so far. As a practical technique to regulate the surface tension of RTLM, the fundamental principles and applications of electrowetting are also interpreted. Moreover, the unique phenomena of RTLM surface tension issues such as surface tension driven selfactuation, modified wettability on various substrates and the functions of oxides are discussed to give an insight into the acting mechanism of surface tension. Furthermore, future directions worthy of pursuing are pointed out.

Surface tension of liquid metal: role, mechanism and application

The effects of Al and Mn contents on the size, composition, and three-dimensional morphologies of inclusions formed in Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) steels were investigated to enhance our understanding of the inclusion formation behavior in high Mn-Al–alloyed steels. By assuming that the alumina is a dominant oxide compound, the volume fraction of inclusions estimated from the chemical analysis, i.e., insoluble Al, in the Fe-Mn-3Al steels was larger than the inclusion volume fractions in the Fe-Mn-1Al and Fe-Mn-6Al steels. A similar tendency was found in the analysis of inclusions from a potentiostatic electrolytic extraction method. This finding could be explained from the terminal velocities of the compounds, which was affected by the thermophysical properties of Fe-Mn-Al steels. The inclusions formed in the Fe-Mn-Al–alloyed steels are classified into seven types according to chemistry and morphology: (1) single Al2O3 particle, (2) single AlN or AlON particle, (3) MnAl2O4 single galaxite spinel particle, (4) Al2O3(-Al(O)N) agglomerate, (5) single Mn(S,Se) particle, (6) oxide core with Mn(S,Se) skin (wrap), and (7) Mn(S,Se) core with Al2O3(-Al(O)N) aggregate (or bump). The Mn(S,Se) compounds were formed by the contamination of the steels by Se from the electrolytic Mn. Therefore, the raw materials (Mn) should be used carefully in the melting and casting processes of Fe-Mn-Al–alloyed steels.

Characterization of Nonmetallic Inclusions in High-Manganese and Aluminum-Alloyed Austenitic Steels

Following a series of laboratory-scale experiments, the mechanism of a chemical reaction $4[\rm{Al}] + 3(\rm{SiO}_2) = 3[\rm{Si}] + 2(\rm{Al}_2\rm{O}_3)$ between high-alloyed TWIP (TWin-Induced Plasticity) steel containing Mn and Al and molten mold flux composed mainly of CaO-SiO2 during the continuous casting process is discussed in the present article in the context of kinetic analysis, morphological evolution at the reaction interface. By the kinetic analysis using a two-film theory, a rate-controlling step of the chemical reaction at the interface between the molten steel and the molten flux is found to be mass transport of Al in a boundary layer of the molten steel, as long as the molten steel and the molten flux phases are concerned. Mass transfer coefficient of the Al in the boundary layer ($k_{\rm{Al}}$) is estimated to be 0.9 to 1.2 × 10−4 m/s at 1773 K ($1500\,^{\circ}$C). By utilizing experimental data at various temperatures, the following equation is obtained for the $k_{\rm{Al}}; \ln k_{\rm{Al}} = -14,290/T - 1.1107.$ Activation energy for the mass transfer of Al in the boundary layer is 119 kJ/mol, which is close to a value of activation energy for mass transfer in metal phase. The composition evolution of Al in the molten steel was well explained by the mechanism of Al mass transfer. On the other hand, when the concentration of Al in the steel was high, a significant deviation of the composition evolution of Al in the molten steel was observed. By observing reaction interface between the molten steel and the molten flux, it is thought that the chemical reaction controlled by the mass transfer of Al seemed to be disturbed by formation of a solid product layer of MgAl2O4. A model based on a dynamic mass balance and the reaction mechanism of mass transfer of Al in the boundary layer for the low Al steel was developed to predict (pct Al2O3) accumulation rate in the molten mold flux.

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A Reaction Between High Mn-High Al Steel and CaO-SiO 2 -Type Molten Mold Flux: Part II. Reaction Mechanism, Interface Morphology, and Al 2 O 3 Accumulation in Molten Mold Flux

The distribution of silicate anionic species (Qn units, n=0, 1, 2, 3) and the chemical speciation of oxygen in CaO-SiO2-MO (M=Mn and Mg) slags were investigated by micro-Raman spectroscopic analysis. Furthermore, the thermochemical properties were evaluated using a concentration of free oxygen and a degree of polymerization. A good linear relationship was obtained between sulfide capacity and concentration of free oxygen in the CaO-SiO2 (-MnO) melts at 1500 to 1600 °C. However, even though there was more abundant free oxygen in the CaO-SiO2-MgO system than in the CaO-SiO2 system, the sulfide capacity of the former was lower than the latter, indicating that the sulfur dissolution behavior in the silicate melts cannot be simply explained by the content of free oxygen, because the composition dependency of the stability ratio of oxygen and sulfide ions should be taken into account. The excess free energy of CaO, MgO and MnO linearly decreased as the ln (Q3/Q2) increased. The effect of the degree of polymerization on the excess free energy of mixing of MgO-containing slag was larger than that of MnO-containing slag, which was explained by the difference of the ionization potential between Mn2+ and Mg2+ ions.

Effect of silicate structure on thermodynamic properties of calcium silicate melts: Quantitative analysis of Raman spectra

The temperature dependence of surface tension and density for Fe–Cr–Mo (AISI 4142), Fe–Cr–Ni (AISI 304), and Fe–Cr–Mn–Ni TRIP/TWIP high-manganese (16 wt% Cr, 7 wt% Mn, and 3–9 wt% Ni) liquid alloys are investigated using the conventional maximum bubble pressure (MBP) and sessile drop (SD) methods. In addition, the surface tension of liquid steel is measured using the oscillating droplet method on electromagnetically levitated (EML) liquid droplets at the German Aerospace Centre (DLR, Cologne). The data of thermophysical properties for Fe–Cr–Mn–Ni is of major importance for modeling of infiltration and gas atomization processes in the prototyping of a “TRIP-Matrix-Composite.” The surface tension of TRIP/TWIP steel increased with an increase in temperature in MBP as well as in SD measurement. The manganese evaporation with the conventional measurement methods is not significantly high within the experiments (∆Mn < 0.5 %). The temperature coefficient of surface tension (dσ/dT) is positive for liquid steel samples, which can be explained by the concentration of surface active elements. A slight influence of nickel on the surface tension of Fe–Cr–Mn–Ni steel was experimentally observed where σ is decreased with increasing nickel content. EML measurement of high-manganese steel, however, is limited to the undercooling state of the liquid steel. The manganese evaporation strongly increased in excess of the liquidus temperature in levitation measurements and a mass loss of droplet of 5 % was observed.

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Surface tension and density data for Fe–Cr–Mo, Fe–Cr–Ni, and Fe–Cr–Mn–Ni steels

The density and surface tension of liquid Fe-Mn alloys were determined by using the sessile drop method at 1823 K (1550 °C). The density of liquid Fe-Mn alloys decreased with increasing Mn content. When the molar volume was plotted with respect to the mole fraction of Mn, a linear relationship was obtained. Consequently, it was found that there is no excess volume in liquid Fe-Mn alloy. The surface tension of liquid Fe-Mn alloys was found to decrease with increasing Mn content. The current experimental data were higher than the reported results but close to the calculated results using Butler’s equation.

Density and Surface Tension of Liquid Fe-Mn Alloys

Density measurements of CaO–MnO–SiO2 slags were carried out using the Archimedean method. The density decreased with increasing temperature, and the negative temperature coefficient (–dρ/dT) was in the range of 1.46–3.10 × 10 kg/m K. At a fixed CaO content (25 wt%) and at a fixed SiO2 content (30 wt%), the density of CaO–MnO–SiO2 slag increased with increasing MnO content. The molar volumes of CaO, MnO and SiO2 at 1 773 K were estimated 21.0, 17.2, and 28.0 × 10 m/mol, respectively. The molar excess volume was expressed by , where Ω = –8.97 × 10 m/mol at 1 773 K. The thermal expansion coefficient decreased with increasing XSiO2 because of the enhanced silicate network structure.

https://www.jstage.jst.go.jp/article/isijinternational/52/12/52_2145/_pdf

Density Measurements of CaO–MnO–SiO2 Slags

The density of liquid Fe-Mn-C alloys was determined at temperatures ranging from 1687 K to 1834 K (1414 °C to 1561 °C) by using the advanced sessile drop method. Regression analysis of the experimental data produced the following description of the density of the Fe-Mn-C alloys:
 $$ \begin{gathered} \rho_{{{\text{Fe-}}{\text{Mn}} - {\text{C}}}} \left[ {{\text{g}}/{\text{cm}}^{3} } \right] = \left\{ {\left( {7.10 - 2.2 \times {\text{WFM}} + 0.60 \times {\text{WFM}}^{2} } \right)} \right. \hfill \\ \left. { - \left( {0.129 + 0.67 \times {\text{WFM}}} \right) \times \left[ {{\text{wt}}\;{\text{pct}}\;{\text{C}}} \right] + \left( {1.45 \times 10^{ - 2} + 7.6 \times 10^{ - 2} \times {\text{WFM}}} \right) \times \left[ {{\text{wt}}\;{\text{pct}}\;{\text{C}}} \right]^{2} } \right\} \hfill \\ - \left\{ {1.83 \times 10^{ - 3} - \left( {4.11 \times 10^{ - 5} + 1.7 \times 10^{ - 6} \times {\text{WFM}}} \right) \times \left[ {{\text{wt}}\;{\text{pct}}\;{\text{C}}} \right]} \right\} \times \left( {T - 1823} \right) \hfill \\ \end{gathered} $$
 where
 $$ {\text{WFM}} = \frac{{\left[ {{\text{wt}}\;{\text{pct}}\;{\text{Mn}}} \right]}}{{\left[ {{\text{wt}}\;{\text{pct}}\;{\text{Mn}}} \right] + \left[ {{\text{wt}}\;{\text{pct}}\;{\text{Fe}}} \right]}} $$

Density of Liquid Fe-Mn-C Alloys

The influences of temperature, manganese, and carbon on the surface tension of liquid ternary Fe–Mn–C systems were investigated. The measurements were carried out with the sessile drop method in the temperature range of 1653–1834 K. The manganese and carbon contents were changed between 4.79–9.89 and 1.07–4.20 wt%, respectively. It was demonstrated that the surface tension varied as a linear function of temperature for all the examined samples. With increasing manganese content, the surface tension decreased. When the weight fraction of manganese with respect to iron was fixed, the surface tension decreased with increasing carbon content. From thermodynamic consideration, it was considered that carbon preferentially adsorbed on the metal surface as carbon atoms rather than manganese carbide.

Le Thu Hoai

Papers

Density and Surface Tension of Liquid Fe-Mn Alloys

Density Measurements of CaO–MnO–SiO2 Slags

Density of Liquid Fe-Mn-C Alloys

Effect of surface adsorption of carbon on the surface tension of liquid Fe–Mn–C alloys