Fundamentals and Manipulation of Bare Droplets and Liquid Marbles as Open Microfluidic Platforms

Microfluidics, as one of the most valuable analytical technologies of this century, has played an important role in various fields. Particularly, out-of-channel microfluidics, often referred to as open microfluidics (OMF) has recently drawn wide research attention among scholars for its great potential in convenient manual intervention. Much recent research has been geared toward bare droplets and particle-armed droplets (namely liquid marbles, LMs), which could serve as independent systems in OMF. Their versatile applications include but are not limited to nanomaterials preparation, energy harvesting, cell culture and environment monitoring. These applications are mainly attributed to the excellent independence, low reagent consumption and short reaction time of separate droplets and LMs. In addition, more operation features, such as diverse handling options, flexible controllability and high precision, further enable droplets and LMs carrying small liquid biochemical samples to be manipulated in an open environment freely. Considering the emergence of important research on bare droplets and LMs, this paper systematically reviews the state of the art in the fundamentals and manipulation of the two novel platforms under the frame of OMF. First, the intrinsic property of bare droplets on solid substrates, especially on superhydrophobic ones, is discussed, followed by the formation mechanism of nonwetting LMs and the effect of coating particles on LMs' performance. Then, friction obstacles and actuation principles raised in driving droplets and LMs are further analyzed theoretically. Subsequently, several classical types of manipulation tasks for both droplets and LMs, namely transportation, coalescence, mixing and splitting, are discussed with a focus on key techniques to accomplish the tasks aforementioned. Finally, the fundamental and manipulation similarities and differences between bare droplets and LMs are summarized and future developments towards droplet- or LM-based microreactors and microsensors are recommended accordingly.