Viscous effects on the interaction of granular particles with floating oils in water

Science of the Total Environment 628–629 (2018) 835–839

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Short communication

Viscous effects on the interaction of granular particles with floating oils in water Berrin Tansel ⁎, Daria Boglaienko Florida International University, Civil and Environmental Engineering Department, 10555 West Flagler Street, Engineering Center, Miami, FL 33174, USA

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Light hydrophobic liquids (LHLs) can be submerged in water by granular particles. • Interaction of silicone oils with different viscosities was tested with sand particles. • Silicone oils submerged by forming granular encapsulated oil sacks in water. • Encapsulation coverage was highly dependent on the granular particle size. • Size of the submerged oil sacks increased with increasing oil viscosity.

a r t i c l e

i n f o

Article history: Received 26 November 2017 Received in revised form 7 January 2018 Accepted 10 February 2018 Available online xxxx Editor: Jay Gan Keywords: Floating oil Granular particles Hydrophobic liquids Granular encapsulation Liquid marbles Oil submergence

a b s t r a c t Light hydrophobic liquids (LHLs) can be submerged in water with granular particles by forming particle encapsulated liquid sacks. Formation and submergence of granular encapsulated LHL sacks can be an effective method for capturing and controlling the fate of floating oils. However, formation characteristics of the LHL sacks and effect of LHL viscosity on their behavior are not well understood. In this study, we examined the encapsulation characteristics of LHL sacks depending on liquid viscosity. Silicone oils with viscosities ranging from 10 cSt to 1000 cSt were used as the LHLs. Sand with two different particle sizes (40–100 mesh and 20–30 mesh) were used as the granular particles. The submerged LHL sacks were stable and remained separate from each other without collapsing or aggregating over time. They could be moved in water by sliding while keeping their encapsulation. © 2018 Published by Elsevier B.V.

1. Introduction In recent years, particle stabilized interfaces have received attention for development of structured materials and novel applications (Binks and Murakami, 2006; Bormashenko et al., 2012; McHale and Newton, ⁎ Corresponding author. E-mail address: tanselb@fiu.edu (B. Tansel).

https://doi.org/10.1016/j.scitotenv.2018.02.124 0048-9697/© 2018 Published by Elsevier B.V.

2015). The particle stabilized liquid interfaces have been used in droplet form (e.g., ink jet printing), as well as encapsulation of different types of liquids with nanoparticles. Applications in spray form using atomization or jetting techniques by dispersion of liquid phase into small droplets (ranging from submicron to several hundred microns in diameter) are of interest in industrial applications such as agriculture, printing, painting, liquid fuel injection, where spreading of liquid in air (e.g., fertilizer application) or fine coating of liquid on a solid surface (e.g., ink jet

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Table 1 Properties of the silicone oils used (at 25 °C). Kinematic viscositya (cSt)

Specific gravitya (g/cm3)

Surface tensionb (mN/m)

Interfacial tension with waterc (mN/m)

10 50 100 1000

0.930 0.959 0.965 0.970

20.1 20.7 20.9 21.2

45.8 46.6 46.2 46.0

a

Source: http://www.powerchemical.net/library/Silicone_Oil.pdf. Source: Sigma Aldrich http://www.sigmaaldrich.com/materials-science/material-science-products.html?TablePage=20204397. c Source: G.A. Padron Aldana (Padron, 2004) http://drum.lib.umd.edu/bitstream/handle/1903/2160/umi-umd-2140.pdf; jsessionid=08DC914F51D799E2BC5E92774A022D7F?sequence=1. b

printing on paper) is required. Applications with much larger drops (or liquid sacks) of hydrophobic liquids ranging from several millimeters to a few centimeters in water are of interest for potential applications for capturing and controlling the fate of oils after release to the marine environment or industrial applications (Abkarian et al., 2013; Boglaienko and Tansel, 2015; Boglaienko et al., 2016). The liquid globules which can transport a small amount of liquid on a hydrophobic solid surface have recently been called liquid marbles due to their quasi spherical shapes, soft solid characteristics, and reduced adhesion to the solid surface (Aussillous and Quere, 2006; Bergeron, 2003; Bormashenko et al., 2010; Tavacoli et al., 2012; Fernandes et al., 2014). The same underlying interfacial energy principles at the micro scale also allow larger particles (i.e., granular) to encapsulate floating oils into stable globules to form granular

Fig. 1. Granular particle encapsulated oil sacks in aqueous medium after submergence. a. Surface coverage with small and large sand particles (sand: grain size 0.15–0.42 mm (left), 0.60–0.85 mm (right)); silicone oil viscosity: 1000 cSt (left), 10 cSt (right). b. Sand encapsulated silicone oil in water (sand: grain size 0.15–0.42 mm, silicone oil viscosity: 100 cSt). The red circles delineate the sand layer outside the oil sack. c. Oil sack elongation due to air bubble formation; shape of encapsulated sack before (left) and after (right) the air bubble is released (sand: grain size 0.15–0.42 mm, silicone oil viscosity 50 cSt). d. Comparison of oil sacks formed with silicone oils with different viscosities (with sand particles 0.60–0.85 mm) left to right: 10, 50, 100, 1000 cSt; 50 cSt: sack elongation due to larger air bubble. e. Comparison of oil sacks formed with silicone oils with different viscosities (with sand particles 0.15–0.42 mm) left to right: 10, 50, 100, 1000 cSt. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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In order to study the effect of LHL viscosity on formation of granular encapsulated oil sacks, we used silicone oil with viscosities ranging from 10 cSt to 1000 cSt and sand with two different particle sizes. Silicone oil was obtained from Sigma-Aldrich. Pure quartz sand with fine particle size (40–100 mesh; 0.42–0.15 mm particle size, average diameter 0.28 mm; referred as fine sand) was obtained from Acros Organics; pure quartz sand with medium particle size (20–30 mesh; 0.85–0.60 mm particle size, average diameter 0.72 mm; referred as medium sand) was obtained from Spectrum Chemical MFG. The sands were not oven-dried but used in their original condition. Table 1 presents the characteristics of the silicone oils. Experiments were performed using a glass tank (5.68-L capacity, with depth: 16 cm, length: 30.48 cm, width: 10.9 cm) which was divided into four equal compartments (16 cm × 7.62 cm × 10.9 cm) with removable partitions. The glass tank was filled with water almost to the rim and partitions were inserted to form the compartments. The thickness of the floating oil layer was controlled by using a funnel with a cylindrical section at the bottom which submerged in the water. Silicone oil (0.5 mL) was added using 5 mL graduated pipette into the cylindrical section of the funnel (which provided an oil thickness 1.6 ± 0.1 mm). Sand (1.0 g) was added directly and all at once to the floating oil (inside the funnel) from a distance of 5 cm above the liquid surface. All experiments were carried out at the constant room temperature 25 °C. Photographs were taken by a Canon EOS 6D camera using EF24–400 mm f/4.5–5.6 L IS II USM lens. Instantaneous submergence of oil sacks allowed the experiments to be conducted rapidly (within 2–5 s) as the oil globules descended in the tank (16 cm depth). Experiments were conducted in duplicates for each condition. 3. Results and discussion We observed that viscosity of the silicone oil affected the size, shape, and mobility characteristics of the submerged LHL sacks, which had quasi spherical shapes and different encapsulation coverage. Depending on size of the sand particles and the viscosity of oils, LHL sacks were encapsulated either partially or entirely by the granular particles which were positioned at the oil-water interface (Fig. 1). The size of submerged LHL sacks varied depending on the oil viscosity. Lower viscosity silicone oils formed smaller globules, while higher viscosity silicone oils formed larger globules (Fig. 1d, e). This is due to the lower stretching ability of low viscosity oils, which makes it easier to detach from the floating phase (without stretching), hence, forming smaller oil globules. We used the dimensionless Galilei number (Ga) to demonstrate the relationship between the diameter of the LHL sack and the dominance

gD3 n2

Ga ¼

ð1Þ

where g is acceleration due to gravity; D is the diameter of the encapsulated sack; and n is the kinematic viscosity of silicone oil. Lower Ga numbers (which were obtained with higher viscosity silicone oils) indicate the higher effect of viscous forces on the encapsulation of silicone oil (Fig. 2a). The dimensionless aggregation number (AN) was developed in earlier studies to relate the physical properties of the hydrophobic liquid to the amount of liquid encapsulated by granular particles (Boglaienko and Tansel, 2016a). In our case, when kinematic viscosity of the LHL is higher than the kinematic viscosity of water (nHL N nw), AN can be calculated by the following equation: ρHL γ air nHL log ρw γ w nw

AN ¼

ð2Þ

where, ρHL and ρw are density of the hydrophobic liquid and water, γair is the surface tension of LHL, γw is the interfacial tension of LHL with water; nHL and nw are kinematic viscosities of LHL and water, respectively. The values of kinematic viscosities of silicone oils, as well as density, surface tension and interfacial tension of oils, are given in Table 1. Density and kinematic viscosity of water were taken as 1000 g/cm3 and 10−6 m2/s, respectively. The aggregation number does not account for the size and type of granular material; it relates only the physical properties of the hydrophobic liquid. The larger AN values indicate higher amounts of hydrophobic liquid to be submerged with granular particles. As shown in Fig. 2b, silicone oils with higher viscosities had larger submerged oil sacks, thus, led to larger amounts of oil to be captured by the granular particles (with both small and medium size sand).

100000

a

10 cSt

10000 Ga

2. Materials and methods

of either the gravitational or viscous forces (Fig. 2a). The Galilei number is expressed by the following equation (Shires, n.d.):

50 cSt

100 cSt

1000 100 1000 cSt

10

fine sand medium sand

1 0

5 10 15 Average diameter of oil sack (mm)

20

1.50

b

1000 cSt

1.25 1.00 AN

encapsulated oil sacks in water. However, depending on the characteristics of the light hydrophobic liquid (LHL) and the granular particles, both the shape and the mobility characteristics of oil sacks are very different from those of liquid marbles that form on solid surfaces exposed to air. Physical properties of hydrophobic liquids (i.e., density, viscosity, and surface and interfacial tension) affect interaction of LHLs with granular particles (Boglaienko and Tansel, 2016a; Bormashenko et al., 2013). Particle characteristics (i.e., particle size, wetting characteristics, and surface morphology) have been reported to correlate with aggregation characteristics (Boglaienko and Tansel, 2016b; Nguyen et al., 2010), capillary cohesion (Domenech and Velankar, 2015) and electrostatic forces (Boglaienko and Tansel, 2017) which control the positioning of the granular particles at the LHL-water interface. The objectives of this study were to evaluate the effects of oil viscosity on the encapsulation characteristics, shape, and air entrapment within the macro scale oil sacks formed with sand particles in silicone oil-water systems.

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100 cSt

0.75

50 cSt

0.50

10 cSt

0.25 0.00 0

5 10 15 Average diameter of oil sack (mm)

20

Fig. 2. Ga and AN numbers for the granular encapsulated silicone oil sacks and their relation to diameter of the submerged oil sacks. Error bars are based on the data from duplicate experimental runs.

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Majority of the oil sacks had a distinctly identifiable air bubble (Fig. 1b, c) which was caused by air entrapment during rapid addition of the granular particles, and was spontaneously released over time. Entrapment of air and emergence of air bubble resulted in elongation of the submerged oil sacks (Fig. 1c, d, e). After the air bubble was released, the shape of the globules was stable. Impact upon contact with the bottom of the tank caused deformation of the oil sacks from nearly spherical to oblong shape. Also, an air bubble was released from the oil sacks upon contact with the bottom. With the high viscosity oils, deformation after contact with the bottom was higher and took longer time for the shape to stabilize. This is because the viscosity affected the conversion of some of the kinetic energy by deformation (Aussillous and Quere, 2006; Bergeron, 2003; Caviezel et al., 2008). The more viscous the oil, the larger the dissipation of kinetic energy upon impact, and the smaller the elasticity of the oil sack (Aussillous and Quere, 2006). A second air bubble formed in most of the submerged oil sacks. The appearance of the second air bubble was slower, especially with the higher viscosity oils. Submerged silicone oil sacks remained separate from each other, and could be moved in water (by sliding) while keeping their granular encapsulation.

Sand particles with larger diameters (medium sand) provided smaller coverage, while particles with smaller sizes (fine sand) covered a larger fraction of the submerged oil globule. The granular encapsulation coverage was most noticeably different in the cases with fine sand using1000 cSt silicone oil and medium sand using 10 cSt silicone oil (Fig. 1a). Although the size of the oil sacks increased with increasing oil viscosity, the oils sacks initially covered the over 80% of the oil-water interface with fine sand. On the other hand, the encapsulation coverage was less that 50% of the oil-water interface at all oil viscosities with medium sand. While passing through the floating oil layer, the granular particles were coated with oil, making them hydrophobic, hence, limiting their interaction with the water phase. The hydrophobic particles accumulated at the oil-water interface, resulting in densification of the floating oil layer; which allowed some floating oil to eventually separate from the floating phase and submerge into the water column forming granular encapsulated oil sacks. Initially, the submerging oil sacks had more sand entrapped within the oil than necessary to make them to submerge by gravitational forces. This was due to the viscosity of the oil which retained the particles. Over time, some sand was released from

a

b

c

d

5 mm

2.5 mm 5 mm

2.5 mm Fig. 3. Gradual movement and partial release of sand particles from oil-water interface resulting in shape change. a. Shape transformation of oil sack after stabilization of the encapsulation (silicone oil: viscosity 100 cSt, sand: grain size 0.15–0.42 mm). b. Granular slip of the large particle size encapsulation and sack's transformation to ovoid with encapsulation coverage being reduced to less than 50% of the oil-water interface. c. Granular slip of the small particle size encapsulation and sack's transformation to ovoid with encapsulation coverage over 50% of the oil-water interface. d. Formation of tightly packed encapsulation observed with both medium sand (top) with close up view of the staggered arrangement of sand particles (top right) and fine sand (bottom) with close up view of the staggered sand particles (bottom right).

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the encapsulation, especially for the oil sacks with the larger diameter and those formed with larger size of granular material. This resulted in a smaller area to be encapsulated (at the lower portion of the submerged oil sacks) when medium sand was used, hence, causing vertical elongation of the sack due to lower density of the oil than water (i.e., buoyancy effects). Sand particles were released from the oil globule by several mechanisms: (1) impact upon contact with the bottom, (2) reduction in surface area during shape recovery (i.e., elongation), (3) slipping of particles at the interface by gravity and compression, hence, increasing stress on the particles near the bottom, (4) release of air bubble which resulted in decrease in size (both volume and surface area) of the submerged oil sack. Once the oil sack reached the bottom, the sand particles moved at the water-oil interface by gravity, gradually forming a stable encapsulation which remained in the lower portion of the submerged oil sacks (Fig. 3a). As the particles formed the single layer stable encapsulation in the lower portion of the submerged sack, the oil sack became elongated vertically due to lower density of the oil, lack of granular particles pressing down on the upper portion of the sack and reduced particle coverage density on the upper portion (Fig. 3). After the stabilization of the encapsulation, oil sacks remained submerged and did not float back to the water surface. Higher viscosity oils initially were able to hold extra amount of sand due to elongation. However, a large amount of sand was released during descend of the oil sacks in water. The granular encapsulations formed at the oil-water interface were observed to be tightly packed due to the combined effects of gravitational and capillary forces with both fine and medium sand. The close up examination of the particles in the encapsulation showed that some particles were pushed out forming staggered arrangements, however, remaining at the interface. The staggered arrangement of the particles due to compression resulted in formation of a very tightly packed encapsulation layer at the interface (Figs. 1 and 3d). The tightly staggered granular packing characteristics observed are different from those reported with the nanoscale particles (Bormashenko, 2011) or colloidal particles (Aveyard et al., 2003) where gravitational forces are relatively less significant and electrostatic forces limit the ability of the particles to form such tightly packed arrangements. In this study, we evaluated on the effect of viscosity on the detachment and submergence silicone oil from the floating layer with granular particles. Other factors such as interfacial tension, particle characteristics and particles addition rate (e.g., momentum of the particles when they come in contact with the floating oil layer) could also affect the detachment characteristics and number of oil sacks forming. 4. Conclusions Effects of oil viscosity and granular particle size on submergence in water and formation of granular encapsulated oil sacks in water were evaluated experimentally. The granular particles remained attached to the submerged oil globules by forming an encapsulation layer at the oil-water interface. As the granular particle size increased, the granular encapsulation coverage decreased due to increasing gravitational effects on the particles. Wetting of the granular particles by the floating oil layer during the initial contact and passage of the particles through the oil layer prior to the water increased their hydrophobicity which resulted in very tight and staggered packing of the granular particles (i.e., compressed packing) at the oil-water interface. The compressed arrangement allowed more particles to remain at the interface than necessary to cover the surface by a single layer of particles arranged at the oil-water interface of the submerged oil sack. Addition of the granular particles near the surface of the floating oil resulted in entrapment of air within the submerged oil sacks, affecting their shape and granular capture characteristics depending on the size of the particles added.

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The observed phenomenon can have applications in biological systems for transporting macro molecules in liquid state (in sack) or particles as encapsulation; and in large scale aquatic environments for capturing floating oils with granular/pulverized particles as well defined liquid sacks. For example, the granular particles can be functionalized so that the oil sacks can work as micro scale reactors (i.e., for oxidation, nutrient release). Formation of macro scale transportable or stationary encapsulated liquid sacks as a separate phase in aqueous environments could allow development of transformative technologies for controlling fate of floating oils released to water bodies. Acknowledgements This research was partially supported by Gulf of Mexico Research Initiative (GoMRI) through funding to Consortium for the Molecular Engineering of Dispersant Systems (CMEDS) under RFP-I: Consortia Grants and Postdoctoral Fellowship by Florida International University. References Abkarian, M., Protiere, S., Aristoff, J.M., Stone, H.A., 2013. Gravity-induced encapsulation of liquids by destabilization of granular rafts. Nat. Commun. 4, 1895. Aussillous, P., Quere, D., 2006. Properties of liquid marbles. Proc. R. Soc. London, Ser. A 462, 973–999. Aveyard, R., Binks, B.P., Clint, J.H., 2003. Emulsions stabilised solely by colloidal particles. Adv. Colloid Interf. Sci. 100-102, 503–546. Bergeron, V., 2003. Designing intelligent fluids for controlling spray applications. C. R. Phys. 4, 211–219. Binks, B.P., Murakami, R., 2006. Phase inversion of particle-stabilized materials from foams to dry water. Nat. Mater. 5, 865–869. Boglaienko, D., Tansel, B., 2015. Instantaneous stabilization of floating oils by surface application of natural granular materials (beach sand and limestone). Mar. Pollut. Bull. 91, 107–112. Boglaienko, D., Tansel, B., 2016a. Encapsulation of light hydrophobic liquids with fine quartz sand: property based characterization and stability in aqueous media with different salinities. Chem. Eng. Sci. 145, 90–96. Boglaienko, D., Tansel, B., 2016b. Gravity induced densification of floating crude oil by granular materials: effect of particle size and surface morphology. Sci. Total Environ. 556, 146–153. Boglaienko, D., Tansel, B., 2017. Preferential positioning and phase exposure of granular particles at hydrophobic liquid-water interface. J. Clean. Prod. 142, 2629–2636. Boglaienko, D., Tansel, B., Sukop, M.C., 2016. Granular encapsulation of light hydrophobic liquids (LHL) in LHL-salt water systems: particle induced densification with quartz sand. Chemosphere 144, 1358–1364. Bormashenko, E., 2011. Liquid marbles: properties and applications. Curr. Opin. Colloid Interface Sci. 16, 266–271. Bormashenko, E., Pogreb, R., Musin, A., Balter, R., Whyman, G., Aurbach, D., 2010. Interfacial and conductive properties of liquid marbles coated with carbon black. Powder Technol. 203, 529–533. Bormashenko, E., Pogreb, R., Musin, A., 2012. Stable water and glycerol marbles immersed in organic liquids: from liquid marbles to Pickering-like emulsions. J. Colloid Interface Sci. 366, 196–199. Bormashenko, E., Musina, A., Whyman, G., Barkay, Z., Starostin, A., Valtsifer, V., Strelnikov, V., 2013. Revisiting the surface tension of liquid marbles: measurement of the effective surface tension of liquid marbles with the pendant marble method. Colloid. Surf. A 425, 15–23. Caviezel, D., Narayanan, C., Lakehal, D., 2008. Adherence and bouncing of liquid droplets impacting on dry surfaces. Microfluid. Nanofluid. 5, 469–478. Domenech, T., Velankar, S.S., 2015. On the rheology of pendular gels and morphological development in paste-like ternary systems based on capillary attraction. Soft Matter 11, 1500–1516. Fernandes, A.M., Gracia, R., Leal, G.P., Paulis, M., Mecerreyes, D., 2014. Simple route to prepare stable liquid marbles using poly(ionic liquid)s. Polymer 55, 3397–3403. McHale, G., Newton, M.I., 2015. Liquid marbles: topical context within soft matter and recent progress. Soft Matter 11, 2530–2546. Nguyen, T.H., Hapgood, K., Shen, W., 2010. Observation of the liquid marble morphology using confocal microscopy. Chem. Eng. J. 162, 396–405. Padron, G.A., 2004. Effect of surfactants on drop size distribution in a batch, rotor-stator mixer. University of Maryland, College Park, MD http://drum.lib.umd.edu/ bitstream/handle/1903/2160/umi-umd-2140.pdf;jsessionid= 08DC914F51D799E2BC5E92774A022D7F?sequence=1 (online, Doctoral thesis). Shires, G.L., d. A to Z guide to thermodynamics: heat & mass transfer, and fluids engineering, Galileo number, Thermopedia. http://www.thermopedia.com/content/798/, Accessed date: 6 January 2018. Tavacoli, J.W., Katgert, G., Kim, E.G., Cates, M.E., Clegg, P.S., 2012. Size limit for particlestabilized emulsion droplets under gravity. Phys. Rev. Lett. 108, 268306.