title: Fundamental Properties of the Liquid Marble Shell
Creators: name: Singha, Pradip
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Liquid marble
subject: microfluidic system
subject: liquid surface tension

description: Liquid marble is a non-wetting droplet coated with micro- or nanometre-sized particles and exhibits excellent versatility as a standalone microfluidic system. Liquid marble, as an emerging platform for digital microfluidics has shown its potential in biomedical applications, cosmetics, and chemical industries. Unique features such as low evaporation rate, low friction, and a porous shell enable liquid marble to be a potential tool for gas sensing, cell culture, and drug delivery. With the growing interest in liquid marble-based applications, it is important to study the fundamentals of the liquid marble shell. Existing works on liquid marble shell discuss only the structure of the shell but detailed research on the liquid marble shell is still lacking. My research work during my PhD candidature focuses on understanding the behaviour and structure of a liquid marble shell at various conditions and how it affects the liquid marble properties. The integrity of a liquid marble depends on the properties of the shell which comprises of encapsulating particles and pores. The size of the particles, properties of the particles, and the number of layers in the shell determines a liquid marble’s macroscopic properties. For instance, multiple particle layers on the shell give a rough shape and fluffy appearance of a liquid marble which brings uncertainties to the effective surface tension values. A monolayered liquid marble has a smoother surface which minimises the error in the effective surface tension measurement. On the other hand, pores in the shell reduce the effective surface tension which affects the evaporation rate of the liquid marble. The behaviour and the structure of a liquid marble shell depend on the preparation method, morphology of the encapsulating particles, and the properties of the core liquid. The details will be discussed in this thesis. We varied the core liquid surface tension to study the liquid marble shell characteristics. We found that the thickness and the mass of a liquid marble shell vary with core liquid surface tension. Surfactant-mediated surface tension reduction of the core liquid allows the encapsulating particles to penetrate more into the core liquid and decrease the shell thickness. The shell thickness decreases significantly with the surface tension whereas the mass remains almost constant. Interestingly, the trend reverses with a further decrease of the surface tension. In this regime, both the thickness and the mass of the shell increase due to interfacial jamming. We understand the contribution of the particle distribution at the shell to its behaviour and the stability of a liquid marble. We also understand that the preparation method is crucial in determining the liquid marble properties. Next, we investigated the effect of the liquid marble preparation method on its effective surface tension. This work attempts to solve the long-standing problem of inconsistent effective surface tension values of liquid marbles. The effective surface tension values reported in the literature are inconsistent due to particle agglomeration, volume of a liquid marble or the preparation method. Moreover, the prevailing liquid marble preparation method is to roll a droplet on a powder bed. We hypothesize that the lack of rolling duration or revolution speed control contribute to inconsistent effective surface tension values and a systematic preparation approach improves the consistency of the effective surface tension values. We (i) determine the effective surface tension using the natural oscillation of a sessile liquid marble and (ii) investigate the effects of liquid marble preparation methods on the effective surface tension for the first time in this thesis. We find that the systematic preparation method improves the consistency of the measured effective surface tension values. Interestingly, higher revolution speed during preparation causes interfacial jamming at the liquid marble shell which decreases the consistency. We studied the liquid marble shell and its effective surface tension but the detailed understanding of the liquid marble shell is still impeded due to its opaque and fuzzy appearance. We used X-ray computerized microtomography (CMT) to generate an image with a visible interface between the core liquid and the shell to overcome this problem. The visible interface facilitates accurate measurement of the shell thickness and consequently the effective surface tension of the liquid marble. We investigate the effect of liquid marble preparation methods and liquid marble volumes on shell thickness and effective surface tension. We found that increasing the revolution speed during liquid marble preparation increases shell thickness. A liquid marble shell has a uniform packing when the revolution speed is 200-300 rpm. We also found that the effective surface tension of liquid marbles decreases with increasing volume. This could be due to a stronger effect of gravitational force for a large liquid marble. We believe that this thesis could provide a new insight into the characterization of liquid marble and open up a new direction of fundamental research of liquid marble and its shell.