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Capillarity: Revisiting the Fundamentals of Liquid Marbles

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Capillarity: Revisiting the Fundamentals of Liquid Marbles Microfluidics and Nanofluidics (2020) 24:81 https://doi.org/10.1007/s10404-020-02385-9 REVIEW Capillarity: revisiting the fundamentals of liquid marbles Pradip Singha1 · Chin Hong Ooi1 · Nhat‑Khuong Nguyen1 · Kamalalayam Rajan Sreejith1 · Jing Jin1 · Nam‑Trung Nguyen1 Received: 11 June 2020 / Accepted: 1 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Liquid marble, an emerging platform for digital microfuidics, has shown its potential in biomedical applications, cosmet- ics, and chemical industries. Recently, the manipulation and fundamental aspects of liquid marbles have been reported and attracted attention from the microfuidics community. Insights into their physical and chemical properties allow liquid marbles to be utilised in practical applications. This review summarises and revisits the efect of capillarity on the formation of liquid marbles and how it afects the efective surface tension as well as their robustness. The paper also systematically discusses the applied aspect of capillarity of the carrier liquid for transporting foating liquid marbles. Keywords Liquid marble · Surface tension · Capillarity · Digital microfuidics 1 Introduction et al. 2017; Samiei et al. 2016). Droplets can be manipulated with active approaches, such as electrowetting (Teng et al. Microfuidics is the science and technology dealing with the 2020), dielectrophoresis (Velev et al. 2003), thermocapil- manipulation of fuid in sub-millimeter/micrometer scale. lary efect (Chen et al. 2005), acoustic vibration (Chen et al. Microfuidics enables the implementation of liquid handling 2017; Zang et al. 2015), and magneto-wetting (Nguyen et al. process on a single device (Whitesides 2006). Microfuid- 2010a). However, evaporation, handling, contamination, ics has a broad range of applications from lab on a chip for and surface modifcation remain current challenges. Liquid point of care diagnostics to cell culture organ on a chip. It marble-based DMF has been utilised as a novel microfuidic is well known that the liquid fow may behave diferently platform to address the abovementioned issues. in the microscale. For instance, in most cases liquid fow is A liquid marble is a non-wetting liquid droplet encap- laminar in the microscale and body forces, such as weight, sulated by micro- or nanoscale hydrophobic particles. The are negligible compared to the dominant surface tension. concept of liquid marble exists in nature in the form of waste The unique microscale behaviours open up subtopics in disposal of galling aphids. The insects could get trapped in microfuidics, leading to applications in chemical engi- their own honey dew waste, but the little aphids tackle this neering, biomedicines, and cosmetics (Bormashenko 2011; problem in a smart way. During defecation, powdered wax McHale and Newton 2011; Nguyen et al. 2019; Nguyen from the anus encapsulates the honey dew droplets. These and Wu 2004; Oliveira et al. 2017; Ooi and Nguyen 2015). encapsulated droplets can roll rapidly and efciently without Recently, digital microfuidics (DMF) has been emerging sticking anywhere. In this way, aphids save themselves from as a liquid handling technology that manipulates discrete drowning in their own sticky waste (Pike et al. 2002). Aussil- droplets (Choi et al. 2012). This technology ofers several lous and Quere were among the frst to initiate the study of advantages over the conventional continuous fow microfu- liquid marbles (Aussillous and Quéré 2001). A liquid marble idics, such as minimum reagent requirement, fast response, serves as a standalone liquid system, as the liquid is physi- and the ability to scale up through parallelisation (Nguyen cally isolated from the surrounding. A liquid marble is robust (Bhosale et al. 2008; Liu et al. 2015) and elastic (Asare- Asher et al. 2015; Bormashenko et al. 2015b; Whyman and * Nam-Trung Nguyen Bormashenko 2015). Furthermore, a liquid marble has low [email protected] friction, controllable evaporation rate (Dandan and Erbil 1 Queensland Micro- and Nanotechnology Centre, Grifth 2009; Sreejith et al. 2018a), better stability (Bormashenko University, 170 Kessels Road, Nathan, QLD 4111, Australia Vol.:(0123456789)1 3 81 Page 2 of 15 Microfluidics and Nanofluidics (2020) 24:81 et al. 2012; Cengiz and Erbil 2013), and unsusceptible to another liquid. The capillary interaction between the parti- contamination (Aussillous and Quéré 2006). An existing cles afects the force balance within the coating of a liquid issue in microfuidics is the lack of control over wetting, marble and modifes its surface tension. The modifed sur- which causes contamination and afects performance. Con- face tension is termed as the efective surface tension of a ventional microfuidic systems overcome such problem by liquid marble (Arbatan and Shen 2011; Bormashenko et al. physical and/or chemical modifying device surfaces. Surface 2009c). The efective surface tension determines the com- topography is one of the physical parameters for wetting pressibility and lifetime of a liquid marble. The lifetime of a control. For example, replication of lotus leaf surface struc- foating liquid marble depends also on the surface tension of tures through soft lithography enables a substrate to achieve the carrier liquid. A foating liquid marble has high mobility Cassie–Baxter wetting (Sun et al. 2005). Surface modifca- due to its non-wetting behaviour and the lower friction with tion with physical vapour deposition have also been widely the carrier liquid surface. The surface tension of the carrier used to manipulate the wetting (Lugscheider and Bobzin liquid can direct the movement of a foating liquid marble, 2001). On the other hand, chemical vapour deposition and which could be utilised as a vehicle for targeted drug deliv- surface functionalisation modify the chemical properties of ery (Kavokine et al. 2016; Paven et al. 2016). On the other the surface to tune the contact angle (Moseke and Gbureck hand, a foating liquid marble induces a negative meniscus 2019). The coating of a liquid marble makes it perfectly non- on the carrier liquid surface. This meniscus depression ena- wetting. While surface modifcation requires complex fabri- bles capillarity-based self-assembly (Singha et al. 2019). cation processes, liquid marbles are simple and already fnd The meniscus angle depends on the size of the liquid marble applications, such as gas sensors (Tian et al. 2010a, b), micro (Ooi et al. 2016), and the surface tension of the carrier liquid reactors (Arbatan et al. 2012; Bormashenko 2017; Sato et al. (Singha et al. 2019). Very few works were reported on the 2015; Nguyen et al. 2020), and chemical/biological assays efect of carrier liquid surface tension on liquid marble. The (Arbatan et al. 2012). Extensive research have been carried following sections discuss in details the efect of the capil- out on tuning the properties of liquid marbles (Aussillous larity on properties and behaviour of liquid marbles. and Quéré 2006; Bormashenko et al. 2009c), forming Janus liquid marbles (Bormashenko et al. 2011b; Xu et al. 2014), transport (Kavokine et al. 2016; Paven et al. 2016), and foat- 2 Efect of surface tension on the formation ing liquid marbles (Bormashenko et al. 2009a, b, 2010a, of a liquid marble 2011a; Gao and McCarthy 2007; Jin et al. 2019, 2020; Ooi et al. 2018; Rao et al. 2005; Tosun and Erbil 2009; Xue A liquid marble consists of a liquid droplet and a coating et al. 2010). Although there are many previous reviews on of micro- or nanosized hydrophobic particles. The surface the fundamental and applied aspects of liquid marbles (Jin tension of the liquid and the surface energy of the particles et al. 2017; Ooi and Nguyen 2015; Sreejith et al. 2018b), interact to minimise the energy of the system. If a solid par- none particularly focuses on the efect of the capillarity. The ticle is attached to the liquid–air interface, the surface free present paper aims to revisit the fundamental and applied energy G of the particle is as follows (Aussillous and Quéré aspects of capillarity in liquid marbles. 2006; Tyowua 2018): Capillarity or capillary action signifes the ability of a 2 liquid to be raised or depressed at the contact line between G = la Ala − R1 + slAsl + saAsa, (1) the liquid and the container. Capillarity is determined by (1) the surface tension of the liquid , (2) the gravitational where R1 = Rp (1 − cosθ)/2, Aij and γij are the interfacial area force, and (3) the contact angle between the liquid and the and the interfacial tension at the i–j interface, respectively, solid surface. The capillary length lc = ∕ g correlates the Rp is the radius of the particle, and θ is the three-phase con- former two factors. Capillary length √is a scaling variable tact angle. refecting the balance between the surface tension force If the particle is fully immersed into the liquid or freely and the gravitational force. The capillary length plays an released into the air, the surface energy will be G1 or G2, important role in determining the shape of a liquid marble, respectively, which is usually quasi-spherical or puddle-like depending G = A + A + A , on its volume (De Gennes et al. 2013). Conversely, capil- 1 la la sl sl sa (2) larity is vital for the formation and stability of a liquid mar- ble, because particles at the liquid–air interface introduce G2 = laAla + sa Asl + Asa . (3) an additional meniscus on the surface of the droplet. The size and properties of the particles afect the surface energy The energy for particle detachment is found by subtract- of the droplet (Nguyen et al. 2010b). These particles trap ing Eq. (1) from Eqs. (2) and (3), such that (Aussillous and air, allowing a liquid marble to foat on the free surface of Quéré 2006; McHale and Newton 2011) 1 3 Microfluidics and Nanofluidics (2020) 24:81 Page 3 of 15 81 ΔG = R2 (1 − cos )2. particles during the rolling process. The wetting transition 1,2 p la (4) at the interface depends on the contact angle of the liquid The energy required for the detachment of a particle from with the particles bed.
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