  Gallium-based liquid metal is getting increased attention in conformal
flexible electronics for its high electrical conductivity, intrinsic
deformability and biocompatibility. A series of flexible devices are developed
based on the micro-particles of liquid metal. But it is still challenging to
fabricate conformal liquid metal film with a large area and high uniformity.
Interfacial self-assembly is a competitive candidate method. Traditional
interfacial self-assembly methods have difficulties assembling liquid metal
particles because the floating state of the high-density microparticles could
be easily disturbed by gravity. Here, we realized the multi-size universal
self-assembly (MUS) for liquid metal particles with various diameters
(0~500{\mu}m). By introducing a simple z-axis undisturbed interfacial material
releasing strategy, the interference of gravitational energy on the stability
of floating particles is avoided. Benefits from this, the ultra-conformable
monolayer film, with large area (>100 cm2) and high floating yield (50%~90%),
can be fabricated by liquid metal particles. Furthermore, the monolayer can be
conformally transferred to any interesting complex surface such as human skin
and plant leaf, to fabricate substrate-free flexible devices. Without
interference from the mechanical response of traditional substrate, the liquid
metal e-tattoo is more user-friendly and can realize feel-less continuous
monitoring.
