Abstract
In this paper, we propose a novel but versatile magnet-actuated platform for droplet manipulation, which uses a ferrofluid film floating on a liquid surface as magnetic actuator. In contrast to the traditional magnetic droplet manipulation, this platform can handle droplets without magnetically functionalizing them. Due to the immiscibility of the oil-based ferrofluid and water, the droplets desired to be manipulated can stably rest on the surface of the floating ferrofluid film (FFF) under the action of surface tension, thereby offering possibilities for magnetically-driven droplet manipulations. Such a floating, magnetically responsive liquid film not only offers an open surface for active 2D droplet manipulation, but also enables complex droplet manipulations in 3D space. Using FFF, we demonstrate a “full-space” droplet manipulation, including droplet transport/coalescence above FFF (i.e. in air), droplet transport/coalescence on FFF and droplet encapsulation/release under FFF (i.e. in liquid). Furthermore, we investigated the effects of the magnetic field intensity, the ferrofluid concentration, the droplet volume, and the FFF thickness on droplet kinematics. By finely tuning these operating conditions, the FFF strategy can enjoy more operational latitude than traditional droplet systems, thus allowing more versatile liquid handling.
Highlights
Only a permanent magnet by utilizing a magnetically responsive flexible film consisting of hierarchical pillars on the surface[30]
Due to the immiscibility of the oil-based ferrofluid and water, the droplets desired to be manipulated can stably rest on the surface of the floating ferrofluid film (FFF) under the action of surface tension, and a liquid-liquid interface is formed between the droplet and the FFF
In contrast to lubricant-impregnated surfaces where an oil film is on a solid substrate[32,33], FFF system containing a floating ferrofluid film on a liquid substrate offers extra degrees of freedom: the droplets settled on FFF can be enwrapped by ferrofluid and dragged upward to a ceiling substrate or dragged downward into the liquid substrate by magnetic force
Summary
Only a permanent magnet by utilizing a magnetically responsive flexible film consisting of hierarchical pillars on the surface[30]. In spite of avoiding the functionalization of droplets with magnetic particles, those magnetic actuation technologies for droplet manipulation require either fine-textured surfaces or complicated surface modifications that tend to increase their cost. All these methods can only manipulate droplets on a planar surface (i.e., in 2D). Due to the immiscibility of the oil-based ferrofluid and water, the droplets desired to be manipulated can stably rest on the surface of the floating ferrofluid film (FFF) under the action of surface tension, and a liquid-liquid interface is formed between the droplet and the FFF. In contrast to existing magnetic droplet manipulation methods, the FFF-based droplet manipulation can perform droplet operations in “full-space”, including droplet transport/coalescence above FFF (i.e. in air), droplet transport/coalescence on FFF and droplet encapsulation/release under FFF (i.e. in liquid), allowing complex tasks with droplets, and providing a versatile system for bioassays
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