Exploring the surface properties of the liquid metals

Zavabeti, A 2018, Exploring the surface properties of the liquid metals, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Exploring the surface properties of the liquid metals
Author(s) Zavabeti, A
Year 2018
Abstract Gallium based liquid metal alloys feature unique properties that set promising pathways for developing future technologies. In contrast to mercury, gallium is considered a non-hazardous option as a liquid metal. A variety of metals can be alloyed into gallium in order to create mixtures with exciting physical and chemical properties. Galinstan is a eutectic liquid metal alloy, consisting 68.5 wt% gallium, 21.5 wt% indium, and 10 wt% tin which possesses a lower (below 0 ºC) freezing point and higher surface tension than that of gallium liquid metal. Surface tension manipulations of liquid metals in electrolyte environments play an important role in the development of their emerging applications in electrochemical systems and microfluidics. However, the effects of electrolytes on the surface tension of liquid metals were not fully explored and an opportunity existed for a comprehensive study of such liquids. Due to significant potential outcomes, exploring the effects of different electrolyte types and concentrations on gallium based liquid metal dynamics was chosen as one of the main foci of this research. In this PhD thesis, the author establishes a new framework to study the effects of different acidic and basic electrolytes on dynamics of galinstan droplets. The droplet is placed in a recess between two microfluidic channels. Each of the channels carries different acidic and basic electrolytes to induce pH gradient across the droplet. Adjusting pH gradient by changing the concentrations of electrolytes on either side of droplets is shown to change the potential of zero charge and the chemical potentials. The author of this thesis hypothesises that according to the Lippmann’s equation this condition should induce surface tension gradient to the droplet. The PhD candidate observes two distinct dynamics of deformation and surface Marangoni flow. To measure the deformation, the author mathematically modelled the change in shape as an aspect ratio between major and minor axes. The surface Marangoni flow is measured by adding micro particles to the electrolytes and tracking their velocities on the surface of the droplet. When low concentrations of electrolytes are used, the hydroxides are less dominant on the surface to enable the deformation of the liquid metal. Instead, when concentrations of both acidic and basic electrolytes are increased, the surface hydroxides are thickened. The formation of thicker surface hydroxides leads to less droplet deformability. Under this condition, the surface tension gradient appears mostly as surface Marangoni flows. The deformation and surface Marangoni flows are replicated and characterised by applying external potentials. The PhD candidate derives theoretical equations for both dynamics which are in good agreement with the experimental results. The author also became involved in the development of applications in low pH gradients when the deformation is the more dominant dynamic. The author discovers that under the deformation dominant conditions, self-propulsion of the droplets is possible. Applications including pumping and switching are demonstrated by only changing the electrolytes surrounding the droplet without the need for an external power supply.
After investigating the surface manipulation of liquid metals using electrolyte environments, this PhD research is extended to explore the fundamental understanding of the oxide compounds that form on the surface of gallium liquid metals. In ambient conditions, self-limiting layers of metal oxides spontaneously form on the surface of liquid metal alloys. These oxide layers are naturally occurring two-dimensional (2D) materials. The author of this PhD thesis demonstrates two novel methods for delaminating the surface oxides. In the first method, surface oxides are directly transferred onto a substrate via a van der Waals exfoliation technique. In the second method, a reactor is devised for the large scale production of the 2D sheets. During this process, compressed air is injected into the liquid metal and the 2D exfoliated sheets are collected in a solvent that is situated above the liquid metal. PhD candidate then investigates the possibility of modifying the composition of the surface oxides. To achieve this, the author of the thesis uses galinstan liquid metal as a reaction solvent and incorporates other metals, such as transition metals, post transition metals and lanthanides. The author hypothesised that the surface oxide composition is dominated by the metal oxide with more favourable Gibbs energy of oxide reaction in comparison to the base alloy. Author uses this method to synthesise the thinnest ever reported 2D hafnium oxide sheets and develops insulating layers for applications in electronics.
In the final stage of this PhD project, the author of this PhD thesis furthers this concept to grow low dimensional materials at the interface of galinstan liquid metal with water molecules. 2D sheet and One-dimensional (1D) fibre morphologies of aluminium oxide compounds are successfully produced by exposing galinstan-aluminium alloy to liquid and vapour phases of water molecules, respectively. Annealing is shown to retain both 1D and 2D morphologies when aluminium oxide hydroxides are converted into aluminium oxides. Both 1D and 2D oxide hydroxides of aluminium feature very large surface areas. The author demonstrates that each of the morphologies have exciting and unique characteristics. 1D morphologies that are grown from the surface of liquid metal are presented to have high transparency. Instead, the 2D grown morphologies are of strong stiffness. Freestanding membranes are made from the self-assembled 2D structures for the filtration of lead contaminated water and oil-water separation. Membranes demonstrate extraordinary high flux rates which author believes to be due to the highly wrinkled structures of the 2D sheets. A green and sustainable synthesis method is presented to reuse liquid metal for many synthesis cycles. The synthesis cycle is shown to repeat several times with negligible loss of liquid metal and a yield of 100%. The synthesis cycles are shown to only require aluminium and water as pre-cursers.
Altogether the author successfully demonstrates several significant discoveries in the course of this PhD research, developed new concepts based on liquid metals and also created a few unique devices with extraordinary properties. It is expected that the outcomes of this PhD research to impact future of many industries including electronics, optics and also filtration.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Materials Engineering not elsewhere classified
Nanotechnology not elsewhere classified
Electrical and Electronic Engineering not elsewhere classified
Keyword(s) low dimensional materials
two dimensional materials
liquid metals
surface sciences
two dimensional metal oxides
free-standing membranes
self- propelling droplets
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Created: Tue, 27 Nov 2018, 15:30:42 EST by Keely Chapman
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