Liquid marbles (LMs) are droplets with volume of typically microliters coated with hydrophobic powder. Liquid core/shell beads are droplets of the same size protected by a solid shell. Liquid marbles are formed by self-assembly of hydrophobic microparticles on the liquid/air interface. Liquid core/shell beads are formed by double emulsion in air or liquid through unique and customised microfluidic systems. The versatility, ease of use and low cost make LMs an attractive platform for digital microfluidics.
The talk will first report our recent discoveries in the physics of liquid marbles (LMs) that allow them to serve as miniature labs. First, a novel LM manipulation technique using dielectrophoresis (DEP) will be reported. The manipulation technique based on DEP suits any material as it only requires a permittivity mismatch between the LM and its surrounding, which exists in almost all practical cases. The DEP setup consists of a small electrode connected to a high direct-current voltage, positioned above the LM to be manipulated. Energising the electrode generates a strong and non-uniform electric field which results in DEP of the LM. LMs with volume up to 25 μL can be lifted, transported and released with this technique. Coalescence is achieved by dropping a lifted LM on another sessile one. Using this manipulation technique, the coalescence of two LMs was demonstrated via vertical collision. With the DEP technique, the distance between LMs and the horizontal offset can be precisely controlled as the optimisation parameters of the coalescence process. A scaling law and an operation map to predict the success rate of coalescence, which allows LMs to serve as practical micro bioreactors or micromixers.
Next, the robustness of LMs was investigated in terms of evaporation and structural deformation. We demonstrated that a LM cluster can survive thermal cycling conditions up to 95°C in a humidity-controlled environment. Survival at elevated temperature makes a LM feasible for its use in polymerase chain reaction (PCR). The use of LMs for PCR was demonstrated with water quality monitoring application on sourcing of microbial contamination.
Finally, the talk will introduce our latest results on core/shell beads. The beads are formed by double emulsion formed in air or liquid using our customised microfluidic devices. The shell is formed by a UV curable polymer and prevents the liquid core to evaporate. In contrast to liquid marble, liquid core/shell beads have a longer shelf life and more suitable for PCR applications. The same manipulation technology developed for liquid marbles can be used for core/shell beads.
The proof-of-concept and basic manipulation techniques reported here indicate a promising future for a new direction in microfluidics research: liquid-bead-based microfluidics. This new technology would allow for minimizing the use of plastic consumables in biological or chemical labs, making our future more sustainable without plastic waste.