title: Fundamental Investigations and Applications of Liquid Marbles
Creators: name: Jin, Jing
Publisher: name: Griffith University
Year: 2020
Subjects: subject: Microfluidics
subject: liquid marbles

description: Microfluidics has been emerging as a feasible platform technology with broad applications in chemical synthesis, food safety, environmental monitoring, and especially, biomedical diagnostics. As a promising off-channel platform with many intrinsic properties compatible with digital microfluidics, liquid marbles (LMs) play an important role for a range of biochemical applications, such as three-dimensional cell culture and polymerase chain reactions of nucleic acids. LMs are micro-scale liquid droplets encapsulated by a protective coating of hydrophobic micro- or nanoparticles. Due to the non-wetting property, LMs can move freely as an individual soft system on various solid and liquid surfaces with low friction. LMs, especially floating LMs, evaporate slower than bare droplets on a superhydrophobic surface while stably keeping their near-spherical shapes. In a LM, the porous solid coating isolates the liquid core from the external environment, which avoids cross-contamination, but enables an excellent permeability towards vapours. These features allow LMs to potentially serve as reservoirs, sensors, pumps and reactors at the microscale for versatile chemical and biological applications. To maximise the potentials and fulfil multiple microfluidic functions in sequence as a separate digital microfluidic platform, LMs need to be manipulated continuously via a reliable actuation scheme. However, no convenient, efficient and cost-effective manipulation method for controllable and automated transport of LMs is yet available. This thesis reports a novel and feasible manipulation technique for LMs, investigates the process of LM coalescence, explores different controlled marble motions, and finally generalises the boundary conditions for actuating LMs in practical applications using dynamic analysis and analytical modelling. In the thesis, the effect of dielectrophoresis was first applied to manipulate the movement of sessile and floating LMs. The dielectrophoretic (DEP) force produced in a highly inhomogeneous direct-current (DC) electric field was utilised to pick up and release sessile LMs to induce marble coalescence. The same method was used to drag, trap and position floating LMs. After a brief introduction to the research project, a systematic literature review on LM coalescence was provided. LM coalescence is essential to understand marble robustness, enabling LMs to serve as micromixers and microreactors. This chapter reviews the state-of-the-art studies focusing on the coalescence process of droplets and, more recently, of LMs. An overview was given on how droplet coalescence and LM coalescence can be induced using external fields. Subsequently, recent developments in manipulation schemes of LMs were discussed based on the nature of actuation energies, and LMs’ diverse applications in various chemical and biological assays were also summarised. The easy actuation and broad applications of LMs enable them to implement more functions in micro total analysis systems. A series of theoretical and experimental works were carried out after these two literature reviews to demonstrate the feasibility of dielectrophoresis for controlled manipulation of LMs. The coalescence process was studied for two identical sessile LMs in vertical collisions aided by DEP handling to elaborate the underlying mechanisms and critical conditions of LM coalescence. By experimentally varying marble volumes, impact velocities and offset ratios, it is concluded that LM coalescence may occur through the coating pore opening mechanism. High-speed imaging was used to investigate the dynamic behaviours of colliding LMs such as the radius change of the liquid bridge between two coalescing marbles, and to derive the generalised conditions of LM coalescence. Furthermore, the DEP method was extended to manipulating floating LMs that move on a free liquid surface with less evaporation. A relatively simple setup was used for dragging floating LMs of various sizes (2.5–30 mL) back and forth across the water surface at high speeds up to 30 mm s􀀀1, allowing for stirring and mixing inside LMs. The manipulation technique and the corresponding analytical model for predicting marble motions reported here potentially facilitate high-throughput and efficient handling of floating LMs containing sensitive biochemical samples. As trapping is essential for efficient sample handling, the investigation on trapping LMs is the key for understanding controlled transport of LM-based digital microfluidic platforms. Due to the simple actuation and the long lifespan, floating LMs are selected as the research object for the dynamic analysis. First, the trapping process of a floating LM utilising dielectrophoresis was experimentally and analytically investigated. Static LMs with volumes up to 50 mL could be effectively trapped by the attractive DEP force from a working distance up to 60 mm under applied voltages ranging from 1.6 to 5 kV. Based on the relationship between the static friction coefficient and the Bond number of floating LMs, operation maps showing critical trapping voltages were derived for successful trapping of the LMs. Next, the two-dimensional DEP trapping process of floating LMs was further investigated. The dynamic behaviours of moving LMs on a free water surface in trapping cases via spiral movements and escaping cases were analysed experimentally and theoretically. More importantly, a governing equation describing successful trapping was generalised from the energy balance. In addition, DEP handling technique could manipulate LMs with a complex electrode configuration. The concept of accurately positioning a floating LM using a pair of identical electrodes is presented. High voltages applied to each electrode generated a non-uniform electric field, which attracted the floating LM towards the corresponding electrode by a DEP force. The combined DEP forces from the electrode pair could accurately position the LM by controlling the voltages separately. The effects of electrode arrangements on positioning capability were also studied by measuring the relative position of the LM to the electrodes at different voltage ratios. An analytical model was formulated to describe the DEP forces of the two-electrode system and a governing equation that determines the LM position for various electrode configurations was then obtained. Besides, the effective working range of this setup and future work with three or more electrodes for positioning a floating LM in two dimensions were discussed. Finally, the thesis concludes with main findings of the research and some perspectives on future studies of LMs. The novel DEP manipulation approach reported in this thesis provides a greater versatility for practical applications of LM-based digital microfluidic platforms. The research outcomes contribute to enhancing the uptake of LM-based digital microfluidics in multidisciplinary research and to broadening the user base of this promising technology. It is expected that the DEP handling technique of LMs will enable the concept of “lab-in-a-marble” with multi-functional microfluidic modules.