Self-Catalyzed Chemically Coalescing Liquid Metal Emulsions

Abstract

Gallium-based liquid metal alloys (GaLMAs) have widespread applications ranging from soft electronics, energy devices, and catalysis. GaLMAs can be transformed into liquid metal emulsions (LMEs) to modify their rheology for facile patterning, processing, and material integration for GaLMA-based device fabrication. One drawback of using LMEs is reduced electrical conductivity owing to the oxides that form on the surface of dispersed liquid metal droplets. LMEs thus need to be activated by coalescing liquid metal droplets into an electrically conductive network, which usually involves techniques that subject the LME to harsh conditions. This study presents a way to coalesce these droplets through a chemical reaction at mild temperatures (T ∼ 80 °C). Chemical activation is enabled by adding halide compounds into the emulsion that chemically etch the oxide skin on the surface of dispersed droplets of eutectic gallium indium (eGaIn). LMEs synthesized with halide activators can achieve electrical conductivities close to bulk liquid metal (2.4 × 10⁴ S cm^-1) after being heated. 3D printable chemically coalescing LME ink formulations are optimized by systematically exploring halide activator type and concentration, along with mixing conditions, while maximizing for electrical conductivity, shape retention, and compatibility with direct ink writing (DIW). The utility of this ink is demonstrated in a hybrid 3D printing process to create a battery-integrated light emitting diode array, followed by a nondestructive low temperature heat activation that produces a functional device.

Keywords: catalysis; chemical coalescence; chemical sintering; conductive inks; emulsions; hybrid 3D printing; liquid metals.

Figure 1

a) Schematic of 3D printable liquid metal emulsion ink. Left: 3D printable emulsion through direct ink writing. Middle (orange outline): the emulsion ink is comprised of dispersed liquid metal droplets. Right (dark red outline): In addition to eGaIn, (i) diphenyl ether (solvent), (ii) dimerized rosin (D140) and (iii) 2‐bromo‐2'‐chloroacetophenone (2B2c) exist in the ink. b) Schematic of chemical coalescence mechanism through 2B2c dehalogenation reaction (1), which releases a hydrohalide (HX), and is subsequently used for chemical etching reaction (2). (c) Dehalogenation reaction (1) of 2B2c occurs in the presence of eGaIn and heat (T = 80 °C) as shown through ¹H NMR. Color of hydrogens in molecular structure correspond to NMR peak with the same color. The orange peak at 4.5 ppm in the heated LME results correspond to phenacyl bromide (Figure S4, Supporting Information), indicate dechlorination. The acetone solvent peak is denoted with a (*). d) Chemical etching of the metal oxide skin (reaction (2)) from the released hydrohalide is responsible for coalescence of eGaIn droplets, as evidenced through XPS spectra data collected on emulsions containing eGaIn, DE and 2B2c.

Figure 2

a) Process for testing formulated inks. Each formulation is shear mixed, stencil patterned on top of copper tapes and heated in an oven for one hour at T ∼ 80 °C. Resistances and 3D scans of each formulation before and after heating is collected to determine conductivity. b) Conductivity of formulated ∼80 v/v% eGaIn emulsions, with each subplot representing a 2B2c activator concentration sweep (0.0–5.5 v/v%) for a concentration of D140 rosin. c) Scanning electron microscope (SEM) images of 80 v/v% eGaIn emulsions before and after heat. The formulations for each emulsion is noted (b) by the symbol in the top left corner of the before heated SEM image. 2B2c activator is needed for eGaIn droplet coalescence and conductivity. Scale bar denotes 60 µm. d) The emulsion retains its shape after being heated: (left) Falsely colored 3D scans to accompany z‐height plot (right). (right) Average 2D height profile of liquid metal emulsion trace before and after heating.

Figure 3

a) Print outcomes of ∼80 v/v% eGaIn liquid metal emulsion inks: Accumulation and discontinuity (red), variable thickness (yellow), and uniform thickness (green). b) Rheology of emulsion ink (0.7 v/v% D140, 4.5 v/v% 2B2c) mixed for 67.5 min at 15.2% relative humidity (RH) showing its printability. Left: viscoelastic behavior of the emulsion ink. The inclusion of eGaIn at high concentration in the emulsion is needed for viscoelasticity. Middle: The relaxation spectrum, which shows the emulsion is predominantly elastic. Right: Flow curve showing shear thinning characteristics of the emulsion. c) Extrusion behavior of emulsion ink (0.7 v/v% D140, 4.5 v/v% 2B2c, mixed for 32.5 min at 21.7% RH) at varying print velocities (V*). Top row: microscope view of print nozzle (scale bars indicate 0.5 mm). Bottom row: resulting filaments at select V*s, printed at 0.5 mm nozzle height. V* = V/C, where V is the print velocity and C is the determined ink extrusion velocity. Scale bars indicate 2 mm. d) Nondimensionalized printed filament width (d/D, where D is the nozzle diameter) vs. V* shows how the printed filament varies with V*. e) Spannability of emulsion ink (0.7 v/v% D140, 4.5 v/v% 2B2c, mixed for 60 min at 16.1% RH). Left: Orthographic view of a triangle containing different spanning features, with inset defining variables used in spanning calculations (d is filament width, L is span length). Right: Height map of printed triangle generated from 3D scan on left. f) z‐deflection (normalized by filament diameter, d) versus x‐coordinate (normalized by length, L) for spanned lines. The length of each spanned line is denoted in lower left of each subplot. Colored data points represent mean z height data along the length of the trace from 3D point clouds; solid black lines represent the model fit of the viscoelastic bending and stretching equation.

Figure 4

a) Automated process of complex device fabrication using liquid metal emulsion ink, using multimaterial printing and automated pick and place for surface mount device components. b) Photographs of assembled LED device (left) before being heated at T ∼ 80 °C, (right) after being heated. After heat, the functioning device can be held up vertically due to the adhesive properties of the coalesced emulsion.