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.