A quantitative framework for the formation of liquid marbles and hollow granules from hydrophobic powders
Graphical abstract
The previously proposed qualitative framework for liquid marble formation was updated in light of new research. The revised framework presents the process steps and quantitative criteria for liquid marble formation via a preformed droplet template as well as new frameworks for the mechanical dispersion regime and for hollow granule formation.
Research highlights
► Presents an updated quantitative framework for liquid marble formation. ► Formation via drop templates or mechanical dispersion are both covered. ► Contact angle and mixing energy are important parameters. ► Framework for hollow granule formation also proposed.
Introduction
“Liquid marbles” are liquid droplets covered with an exterior shell of a hydrophobic powders, and are able to roll and bounce like glass marbles but deform and flex like a fluid [1], [2]. Non stick liquid marbles have attracted significant attention of researchers in the past decade, due to their promising technological applications [3] in biotechnology [4], chemical and mechanical engineering [4], [5], [6], [7], [8], [9], [10], [11], [12], and are already used in several commercial cosmetics [13] (e.g. Dior IOD AquaPowder) and hair care products (e.g. Schwarzkopf Osis powder). Liquid marbles are also promising candidates to be applied in the biomedical and genetic analysis fields where 2D micro fluidics and lab-on-chip methods are used to quickly analyse very small amounts of materials [8], [14], [15]. Liquid marbles, also known as “dry water” literally repel water, bounce off after an impact and slip on a surface. Liquid marbles can be used as micro reservoirs, containing up to 98% w/w water [16] which can move quickly without any leakage as the force needed to move these marbles on solid surfaces is extremely small because of the diminished area of liquid–solid contact [6], [14], [15], [17].
Single liquid marbles can be prepared by rolling or shaking a single drop of fluid on a bed of fine hydrophobic particles [1]. There are now several studies on liquid marble formation at small scale [1], [2], [3], [5], [6], [7], [11], [12], [14], [15], [17], [18], [19] but few studies on large scale production of liquid marbles using conventional granulation equipment are reported in the literature [16], [20], [21], [22].
Aussillous and Quere [1] were the first to report liquid marbles and several studies have focused on the properties of individual liquid marbles, including their robustness and possible methods for setting the marbles in motion [2], [18], electrowetting [15], methane storage [23] and gas sensing [9], [10].
The evaporation rate of the liquid marble is an important parameter to ensure that liquid remains in the marble until it reaches its target location. If the liquid is volatile and evaporates easily, the liquid marble will deform and collapse. The life time of a liquid marble depends on the volatility of the fluid, the chemical nature and particle size of the hydrophobic powder, and the powder wall structure. Gao and McCarthy [19] formed ionic liquid marbles by using fluoride containing polymers (OTFE, PTFE), and these ionic liquid marbles remained floating on a water surface for a week, compared to hydrophobic silica liquid marbles which floated for a minute before coalescing. Dandan and Erbil [14] studied the life time of graphite liquid marbles in comparison with water droplets of the same size. They found that graphite liquid marbles more than twice as long as a pure water droplet.
The liquid marble powder wall hinders evaporation of the droplet but is the most poorly understood contributor to the liquid marble lifetime. Investigation of the internal liquid marble structure using confocal microscopy [24] has shown that the liquid marble wall is composed of a combination of mono and multi layers of particles. Coarse particles (above 50 microns in size) formed mono layers, while fine particles formed a multi-layered powder shell. Bhosale et al. [25] also reported that cohesive powders were more likely to form a multi-layer of particles on the liquid surface. Nguyen et al. [24] found that the extent of penetration of coarse particles into the liquid core was much less than fine particles. Larger particles tend to float on the top of the liquid core instead of submerging into the liquid core.
In our previous paper [26], a preliminary framework for liquid marble formation was proposed for formation of a stable, spherical, liquid marble from a single drop via “solid spreading nucleation” [22]. The final step of the framework postulated that the liquid marble formation was most likely driven by surface energy and could be described by the solid over liquid spreading coefficient, λSL [27]. However, a fresh thermodynamic analysis [28] and calculations [29] of the existing solid–liquid spreading coefficient revealed that the existing solid–liquid spreading coefficient theory is not able to predict whether a liquid marble will form. Instead, a proportional relationship was found between kinetic energy and the percentage of liquid marble coverage, demonstrating that the kinetic energy of impact is responsible for formation of liquid marbles [29]. The shell appears to be formed by a physical flow mechanism during drop impact. The kinetic energy from impact causes an increase in drop surface area due to the drop deformation. The subsequent drop recoil creates fluid flow which entrains the powder and forms the powder shell [30], [31]. Increasing the drop release height (and therefore increasing the kinetic energy of impact) increases the liquid marble powder coverage.
McEleney et al. [32] also found that kinetic energy is required for liquid marble formation, and reported that the surface coverage decreases with increasing particle size. If the particle size is large, the kinetic energy of shaking or rolling the drop may not produce enough kinetic energy to entrain the large particles on the liquid surface. More vigorous agitation (such as mixing in a granulator) would be required for complete surface coverage.
These results supported separate, parallel observations of the role of mixing intensity during large scale production of liquid marbles. In a series of related papers, Forny et al. [16], [21], [33], [34] reported powder encapsulation of water with different level of hydrophobicity fumed silica powder (Aerosil R972, R812S) in two different vertical axis mixers — a “high shear” mixer with knife blades and a “low” shear Triaxe mixer with rotating gyrating paddles. They also concluded that the kinetic energy is the key to liquid marble formation [16]. Forny et al. used two alternate methods to add water to the silica powder: direct loading of the liquid and powder into the mixer prior to commencing mixer operation; and by atomization over 3–4 min. In the low shear Triaxe mixer, atomization was required to form liquid marbles but they collapsed quickly to form a foam or mousse. The high shear mixer was more successful and liquid marbles formed very quickly (approx 10 s). Using the more hydrophobic silica powder (R812S) and high shear processing conditions, up to 98% w/w water was able to be encapsulated with powder in the high-shear mixer. Formation of stable silica covered drops was most effective at the speeds of 12,000 min− 1 or higher in the high shear mixer with knife-edged impeller blades. Below this speed, the water formed a puddle at the base of the bowl and could not be successfully dispersed through the powder. Atomization into the high-shear mixer was not attempted. As the hydrophobicity of the silica powder decreased, the system became less stable, requiring lower impeller speed and more powder to encapsulate the same volume of liquid. Less hydrophobic silica powders were also more sensitive to shear conditions and collapse of the liquid marbles into a foam or mousse was reported [21]. The time for collapse reduced as the impeller speed and applied shear increased.
Binks and Murakami [35] also studied the transformation of particle-stabilized aqueous foam into water-in-air powder (dry water) and vice versa in a system of air, water and fumed silica nano-particles with different degrees of hydrophobicity. The inversion of the air–water–particles system could be achieved in two ways: (i) changing the particle hydrophobicity at constant air/water ratio; or (ii) changing the air/water ratio at fixed particles wettability (Fig. 1). If more fluid is added to a group of liquid marbles (without also adding more powder), the liquid will gradually excluded the air and the resulting phase inversion will create a mousse at high liquid contents.
Forny et al. summarized the investigated the effect of powder hydrophobicity and several process variables on the formation of the powder encapsulated liquid and developed a regime map (which they termed a “state diagram”) [34] shown in Fig. 2, which predicts the formation of three types of final products as a function of the contact angle of the system and the energy per unit mass applied to the process Ep. If insufficient energy is applied during mixing, the liquid and powder remains as two separate phases. The applied energy per unit mass must be above a certain minimum threshold (Ep)min for the liquid to be effectively dispersed through the powder to form liquid marbles. However, further increases in the energy applied per unit mass eventually exceed a maximum energy threshold (Ep)max, which causes the liquid marbles to collapse into a foam or mousse. For liquid marble formation, the energy applied per unit mass Ep should be between (Ep)min and (Ep)max [34]. The quality of final product at very high contact angle (in the right part of the diagram) is uncertain. The map also summarizes that liquid marble formation in highly energetic processes such as high-shear blenders requires highly hydrophobic particles to avoid forced immersion of the particles into the fluid, and while lower energy processes can use moderately hydrophobic particles.
In all of the studies discussed so far, the liquid marbles were the desired end product. Hapgood et al. [22] was the first researcher who dried liquid marbles to form hollow granules at a larger scale although McEleney et al. [32] later reported the formation of a hollow granule shell after polymerising individual poly-methylmethacralate (PMMA) liquid marbles. In both cases, the final product of hollow granules was obtained by removing the interior liquid i.e. the formation of liquid marbles represents an intermediate product. “Designer” granules with a controlled hollow internal structure are ideal for high-value applications in the pharmaceutical, food and cosmetic industries.
Hapgood et al. [22] produced hollow granules in a 2L granulator using a formulation of 70 wt.% of a hydrophobic drug powder, 20% microcrystalline cellulose and 4% hydroxypropyl cellulose binder as hydrophilic excipients plus 1% sodium lauryl sulfate surfactant (added dry to the powder blend). The hollow granules were strong enough to withstand the downstream milling process as the excipients stabilized the powder shell to allow the hollow structure to be preserved during drying.
A follow-up study of large scale production of liquid marbles and hollow granules [20] investigated the effect of liquid to powder mass ratio on the morphology and particle size distribution of hollow granules. Marbles were formed by spraying a HPC binder solution onto hydrophobic silica (Aerosil R202). The optimum L:S ratio was defined as the point where the raw fine particles were granulated but fewer flattened or stretched hollow granules were produced. If the nucleation process starts with a preformed template droplet (e.g. spraying liquid), the final granule size increases and the amount of un-granulated fine particles decreases as the L:S ratio increases. In contrast, Forny et al. [16], [21], [33], [34] used the shear forces generated by the impeller to disperse the liquid through the powder, and they found no effect of L:S ratio on particle size. This implies that the effect of the liquid to solid ratio varies, depending on whether the fluid is atomised to form discrete drop templates, or added in bulk where an agitator disperses the fluid through the powder.
Bhosale et al. [25] investigated the strength of liquid marbles formed with PTFE (7–12 μm) and two types of treated fumed silica powder. High surface area nano-particles (e.g. Aerosil) created more uniform powder shells through uniform coverage of the liquid–vapor interface. These nano-powder shells formed an “elastic” membrane that made the liquid marbles mechanically robust in comparison with conventional liquid marbles, as they were able to withstand higher compressive stresses during drying. This work suggests that nano-powders may be more likely to successfully form hollow granules.
Drying temperature is also an important variable when forming stable hollow granules from a liquid marble precursor [36]. Higher drying temperature, smaller or nano-sized particles and higher binder concentration tended to promote the formation of spherical hollow granules. The hollow granule survival rate was directly proportional to binder viscosity for HPMC and PVP. However for HPC binder, the survival rate was essentially constant regardless of HPC concentration due to precipitation of the HPC binder above the cloud point temperature (45 °C) [36].
This paper updates and validates the qualitative preliminary framework for liquid marble formation proposed previously [26], incorporating the more recent knowledge generated by several recent studies of liquid marble formation. Quantitative criteria for each step are determined and validated with the best data currently available. In addition, we extend the framework to cover the additional required steps for the formation of liquid marbles via mechanical dispersion, linking work by separate research teams, and propose a new qualitative framework for the formation of hollow granules from liquid marbles.
Section snippets
Materials and methods
Glass ballotini spheres (ρ = 2.5g/cm3, Potters Industries Pty Ltd.) in three size grades (AC, AE, and AG/AH with 191, 121, and 65 μm mean particle size, respectively) were used to form liquid marbles. In order to make the glass beads hydrophobic, SIGMACOTE solution (chlorinated organopolysiloxane in heptane, Sigma Aldrich Pty Ltd.) was used. Two additional hydrophobic powders were used: Polytetra fluoroethylene (PTFE) with four different particle size grades (1, 12, 35, and 100 μm, ρ = 2.1 g/cm3,
Droplet and particle size
To form a liquid marble, the preliminary framework [26] proposed that each drop must be much larger than the size of the primary powder particles (i.e. dd ≫ dp) in order for the particle to spread around the droplet template. The question is: to what extent must the droplet size be larger than the particle size? Experimental and literature data were reviewed to find the quantitative correlation between droplet size and particle size. Table 1 summarizes the actual droplet and particle sizes that
Acknowledgements
This project was financially supported by the Australian Research Council under Discovery Projects DP0770462. Scholarship support for N. Eshtiaghi was provided by the Monash Graduate Research School.
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