Liquid Marbles

Example 12.5. The stabilizing effect of powders was impressively demonstrated by making liquid marbles in air [549], Liquid marbles (Fig. 12.11) are obtained by making a small amount of water (typically 1 mm3) roll on a very hydrophobic powder. The powder particles go into the interface and completely coat it so that, after spontaneous formation of the spherical drop, only the solid caps of powder particles come into contact with the solid support. 

Figure 12.11 Liquid marble on a planar solid surface stabilized by a hydrophobic powder. A schematic (left) and a light microscope image (right) are shown. On the right the marble and its mirror image can bee seen. Thanks to David Quere for providing us with the figure.

Synthetic examples of super-hydrophobicity include covers for solar cells, satellite dishes, coatings for papers and microfluidics. A related example is provided by liquid marbles, which are non-stick droplets coated with microscale or nanoscale particles of low surface energy that exhibit very high contact angles (>150°) and extremely low friction when rolling on solid surfaces [37]. Some liquid marbles, coated with hydrophobic particles, will float on a pool of water. An example in nature is the conversion of honeydew droplets into liquid marbles by aphids [38]. 

Liquid marbles are liquid droplets coated with microscale or nanoscale particles. These particles prevent the liquid core from direct contact with supporting substrates, thus creating a unique non-wetting liquid container. Liquid marbles feature many unique properties. They can be merged, divided, and manipulated for various microfluidic applications. 

Liquid Marbles, Fig. 1 The experimental system (Reproduced with permission from Aberle et al. [5])... 

Liquid Marbles, Fig. 2 The evaporation time of liquid marbles and water droplets at different surface temperatures. Liquid marbles feature long evaporation time at all tested temperatures. Water droplets feature the same long evaporation time only when the temperature is beyond the Leidenfrost point. If the temperature is below the Leidenfrost point, water droplets usually evaporate within seconds. A theoretical curve for Leidenfrost phenomenon is also plotted for comparison (Reproduced with permission from Aberle et al. [5])... 

In case of liquid marbles, the observed deformation may very possibly be explained on the basis of Marangoni convection and preferred nucleation. Thermodynamically, because only the bottom layer of particles are directly cooled by the cold substrate, heterogeneous nucleation is... 

Fig. 3 (a) Liquid marble freezing toward a flying saucer-shaped ice marble, (b) The relative change in height (AH/Ho) and diameter (AD/Do) of different-sized liquid marbles with time. The graph delineates that a smaller-sized liquid marble freezes faster compared to a larger marble, (c) A bell-shaped frozen liquid marble. 

Liquid Marbles, Fig. 4 The sketch shows the cross section of the liquid marble with the proposed gap between the freezing front and the coating due to preferential nucleation. The arrow indicates the flow of water to the gap due to Marangoni effects (Reproduced with permission from Hashmi et al. [6])... 

In order to investigate the effects of the absence of coating from the base of a liquid marble on the freezing dynamics, water droplet was... 

Liquid Marbles, Fig. 6 Marble growth followed by rupture upon injecting water at 0.25 mL/min (a-d) and 2 mL/min (e-h) flow rates (Reproduced with permission from Bajwa et al. [7])... 

It is observed that a marble that has not yet reached the stationary rupture point may have already lost its abifity to roll - a key feature of liquid marbles used in many practical applications. Therefore, further experiments were performed to examine the rolling abilities of enlarged liquid marbles. Marbles with initial size of 0.05 mL were allowed to grow by pumping in controlled volumes of water at controlled flow rate and then were tipped to roU on the substrate. In Fig. 7, the marbles which successfully completed an upside-down rotation... 

The heated evaporation of graphite liquid marbles on a superheated substrate was investigated at various surface temperatures and compared with pure water droplets. It is found that if the temperature is above the Leidenfrost point, the evaporation time of liquid marbles and water droplets are almost the same, whereas if the temperature is below the Leidenfrost point, water droplets evaporate much faster and liquid marbles still exhibit the Leidenfrost-like effect. It is postulated that the prolonged evaporation time of... 

In the past decade, liquid marbles has been extensively studied by many research groups worldwide. Most of these studies focused on the fundamental characteristics of liquid marbles, such as the mechanical properties and particle coating dynamics [8, 9]. In recent years, more and more researchers have started to explore novel sensing and actuation mechanisms with liquid marbles [3, 10, 11]. A few proof-of-concept chemical and biochemical liquid marble reactors have also emerged very recently [4]. However, liquid marble research is still in its infancy. Most studies exploring the applications of liquid marbles are at the demonstration... 

Bormashenko E (2012) New insights into liquid marbles. Soft Matter 8 11018... 

Tian J, Arbatan T, Li X, Shen W (2010) Liquid marble for gas sensing. Chem Commun 46 4734-4736... 

Aberle C, Lewis M, Yu G, Lei N, Xu J (2011) Liquid marbles as thermally robust droplets coating-assisted Leidenfrost-like effect. Soft Matter 7 11314-11318... 

Bajwa A, Xu Y, Hashmi A, Leong M, Ho L et al (2012) Liquid marbles with in-flows and out-flows characteristics and performance limits. Soft Matter 8 11604... 

Bormashenko E (2011) Liquid marbles properties and applications. Curr Opin Coll Interface Scl 16 266-271... 

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