**2. Materials and Methods**

### *2.1. Preparation of Galinstan Microdroplets*

Galinstan (68.5 wt.% Ga, 21.5 wt.% In, and 10.0 wt.% Sn) and PDMS (SYLGARD 184) were purchased from Geratherm Medical AG (Geratal, Germany) and DOW (Midland, MI, USA), respectively. EGaIn (75.5 wt.% Ga and 24.5 wt.% In) was purchased from Sigma-Aldrich Korea (Seoul, Korea). To prepare the Galinstan microdroplets, 0.5 g of Galinstan was sonicated in ethanol for 30 min (80 W, 40 KHz). Galinstan was well dispersed during the sonication process and rapidly stabilized by surface oxidation in ethanol, resulting in the suspension of Galinstan microdroplets (<10 μm) as shown in Figure 1a. The same procedure was repeatedly performed for the preparation of the EGaIn microdroplets.

**Figure 1.** (**a**) Preparation of suspensions comprising EGaIn and Galinstan microdroplets by sonication. (**b**) Schematic illustration of the fabrication of flexible solar-blind photodetector using Galinstan microdroplets.

### *2.2. Preparation of Elastomeric PDMS Substrates and Stamps*

For the PDMS substrates, the PDMS precursor, consisting of a silicone elastomer base and a curing agen<sup>t</sup> (in a 10:1 weight ratio), was poured onto a flat Petri dish, and subsequently degassed in a vacuum desiccator for 1 h. The sample was cured at 80 ◦C for 1 h in a convection oven. After thermal curing, the PDMS film was easily peeled off from the Petri dish and then cut into 50 mm × 50 mm specimens. The thickness of each substrate was measured as ~1 mm. Sticky elastomeric PDMS stamps were individually prepared to exfoliate the thin Ga2O3 films. The mixing ratio of the PDMS precursor was modified to a 11:1 weight ratio to delay the saturation of cross-linking and enhance its adhesive properties, and the same preparation procedure as for the PDMS substrates was followed.

### *2.3. Fabrication of Patterned Galinstan Electrodes*

The suspension containing the Galinstan microdroplets was drop-dispensed onto the flat PDMS substrate and then slowly dried at 30 ◦C for 24 h to avoid the formation of structural defects induced by rapid evaporation of the solvent. The dried suspension of the microdroplets in thin-film form was covered with another flat PDMS substrate and subsequently pressed at a pressure of 15 MPa for 5 s to collapse the surface oxide layers and connect Galinstan. After releasing the pressure, the upper PDMS mold was peeled off from the bottom PDMS substrate, resulting in the formation of thin conductive Galinstan films deposited on both PDMS substrates.

The Galinstan films were patterned using a laser marking machine (50 W, Dongil Laser Technology, Gwangju, Korea). The scanning speed of the laser marking machine was 600 mm s<sup>−</sup>1, and the power intensity was 1.0 % of its maximum power (i.e., 0.5 W). A high resolution of 20 μm was achieved in the Galinstan patterning process by this laser ablation method.

### *2.4. Characterization of Flexible and Stretchable Liquid Metal Electrodes*

To investigate characteristics of the liquid metal electrodes under various conditions, thin conductive films (15 × 25 mm2) of Galinstan and EGaIn were individually prepared with the same procedure. The thickness of each film was 1 μm. A semiconductor characterization system (4200-SCS, Keithley, Beaverton, OR, USA) was used for the measurements.

### *2.5. Fabrication of Flexible Solar-Blind Photodetectors Using Ga2O3*

The elastomeric PDMS stamp was brought into contact with the surface of the Galinstan film. A thin Ga2O3 film (<10 nm), which was spontaneously formed on the Galinstan surface, was attached to the sticky PDMS stamp and easily exfoliated from the Galinstan film by peeling off the stamp. The transparent Ga2O3 film on the PDMS stamp was cut and placed between two patterned Galinstan electrodes to complete the device structure.
