2. Results and discussion
The magnetic liquid metal droplet was fabricated by simply mixing and grinding of carbonyl iron particles and Galinstan. Here, we use carbonyl iron particles instead of magnetite particles because they are easier to be wrapped and wetted by liquid metal (Figure S2). Figure 1d shows a photograph of a mortar filled with a mixture of carbonyl iron and particles Galinstan at a mass ratio of 2:8 after mixing. It should be noted that mixing of liquid metal with particles increases the viscosity and may alter the electrical conductivity of the resulting composite. The particle:liquid metal ratio was altered to elucidate the suitable amount of added magnetic material. Figure 1e shows a series of liquid metal droplet robots with different particle and liquid metal mass ratios. At low mass ratios, i.e., 1:8 or 2:8, the composite behaves like a liquid and can be extruded from a syringe. However, at mass ratios greater than 2:8, the composite becomes more solid-like and can only be deformed plastically by exertion of an external force. These composites do not allow triggered break up and coalescence, as in this state, it is challenging to deform the plastic material. Moreover, particle aggregation results in loss of smoothness of the composite surface, which could adversely affect the locomotion of the composite. Furthermore, scanning electron microscopy (SEM) images show that the discontinuity of the composite increases with the proportion of particle in the liquid metal. At high carbonyl iron content, the magnetic particles show significant aggregation (Figure 1f-1j). The energy dispersive spectrometer (EDS) maps in Figure 1k and 1l show the distribution of the carbonyl iron powders and the liquid metal in the mixture. The EDS maps corroborate that at high particle content aggregation of particles is observed, as seen by several particle aggregates discernable in the EDS map, showing the location of Fe. Although a high amount of magnetic particles in the composite increases the force exerted on the composite by a magnetic field, the solid-like nature and increased roughness at mass ratios greater than 2:8 limit the applicability of the composite as shape morphing material. As a compromise between fluidity and sensitivity to an applied magnetic field, a mass ratio of 2:8 (carbonyl iron:Galinstan) was chosen to prepare the magnetic liquid composite.