Spinning Liquid Marble and Its Dual Applications as Microcentrifuge and Miniature Localized Viscometer

Liquid marble offers an attractive droplet manipulation approach by isolating microdroplet in a nonstick encapsulating shell formed via the spontaneous coating of hydrophobic particles onto the liquid surface. While liquid marble prepared using magnetic nanoparticles enables precise spatiotemporal actuation of microdroplets, these manipulations are generally limited to simple and linear spatial maneuver of microdroplets. Herein, we demonstrate the unique and three-dimensional spinning of microliter-sized liquid marble (LM) and its subsequent dual applications as (1) the world's smallest centrifuge and (2) a miniature and localized viscometer. Our LM is responsive to an applied rotating magnetic field, with its spinning speed programmable between 0 and 1300 rpm. This spinning generates an unprecedented centrifugal force of >2g in a LM of ∼1 mm radius. Such centrifugal force facilitates an outward and radial hydrodynamic flow in the enclosed microdroplet, enabling LM to serve as a microcentrifuge for the sedimentation of nanoparticles with >85% separation efficiency. Furthermore, we apply spinning LM as an ultrasensitive spin-to-viscosity transducer to quantify the viscosity of the external suspended liquid in the relative viscosity (η/ η_water) range of 1-70 using ≤1 mL liquid sample. Collectively, the ensemble of benefits offered by spinning LM creates enormous opportunities in the development of multifunctional micromagneto-mechanical devices as promising surface-sensitive microsensor, miniature centrifugal pump, and even microreactor with directed heat and mass transfer mechanism.