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Bai et al. Soft Sci 2023;3:40 https://dx.doi.org/10.20517/ss.2023.38 Page 9 of 34

to be overlooked, surfactants also provide a steric barrier to the system to maintain colloidal stability, where
they also play a key role in many solutions [130,139] .

Mechanical shearing
Mechanical shearing is a straightforward preparation process in which a velocity and pressure difference is
created in the solution by rotating the tool so that the LM is subjected to centrifugal, shear, and tensile
[140]
forces, plus the gravity of the LM itself [Figure 3C] . Under the forces applied, the droplet shears into two
smaller droplets when the droplet deformation exceeds a certain limit (Rayleigh-Plateau limit). By
controlling the rotational speed and choosing the right solution (to obtain different shear forces), droplets
of various sizes can be obtained. Surfactants are also essential for the stability of the NPs during the
[142]
[143]
preparation process; some used in the study are acetic [140,141] , chitosan , and carboxylic acids . As part of
the solution, the surfactant influences the size and morphology of the NPs [Figure 3D i]. In the case of
carboxylic acid, for example, the NPs prepared from surfactants with hydrocarbon chain lengths of eight
(C8) and below show a broad size distribution, while C16 and above show a more concentrated distribution,
which is mainly due to the ordered nature of the chains [Figure 3D i]. In addition, temperature can also
[143]
affect the morphology of the NPs; taking EGaIn as an example, by increasing the temperature, the thickness
of the gallium oxide film can be improved while creating a chemical gradient. When the temperature rises
to a certain value, the gallium oxide film will break down and cause the internal indium-rich fluid to leak
out and form an indium oxide layer on the surface [Figure 3D ii], resulting in a compositional inversion .
[144]

TRANSFORMATION OF LMS IN SENSORS
Physical transformation
Particle sintering
Since the oxide layer on the surface of LMNP is a poor electrical conductor, it is hard to form an effective
conductive path between droplets in sensors. External stimuli are required to break the oxide film so that
the composite can be converted from a dielectric to a conductive state ; this process is known as
[145]
“sintering” [Figure 4A i and ii]. Compared to the sintering method of conventional nanoscale metallic
particles, LM can, by virtue of its liquid nature, rupture the rigid oxide film after being subjected to minor
applied stress, resulting in the LM spilling out and thus forming a circuit , as shown in Figure 4A i and ii.
[146]
This mechanical sintering method is unique to LMNPs, and compared to the sintering methods of
conventional nanometal particle sensors, such as laser sintering and high-temperature sintering , the
[147]
operation is simple and economical. In addition, the metals that make up conventional nanometal sensors,
such as Ag nanowires, have high contact resistance with each other and require high-temperature sintering
[148]
or point-to-point sintering , and similarly, copper particles are sintered with attention to the uniformity
of the connection of the particle necking , while LMs are subject to either mechanical stress (mechanical
[149]
sintering) or thermal stress from the difference in thermal expansion inside and outside the droplet (laser
sintering or thermal sintering), the LM inside the droplet will be completely mixed after the destruction of
the oxide film, forming a circuit without contact resistance, thus achieving lower energy consumption.

Sensing information detection and acquisition
LMNP-filled materials can be used as sensor materials because the low Young’s modulus of LMs makes
them extremely sensitive to external stresses . When the substrate is deformed by the applied stresses, the
[150]
LM will also accordingly deform, inducing changes in the cross-sectional area, conductive paths, and
[151]
number of contact sites , thus causing changes in the electrical signal of the monitoring circuit
[Figure 4A iii]. In contrast to conventional rigid-filled materials, where changes in sensing signals such as
resistance are achieved by the fracture of the substrate when subjected to tension [152,153] , the deformation
[154]
mechanism of the LM causes minimal damage to the substrate and is nearly fully compliant .