Liquid Marbles and Janus Liquid Marbles

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Figure: Principle of the collapse of LMs and LM mediated assembly by modulating the surface tension of the liquid substrate. LM mediated assembly over a pure water surface (top route) and an aqueous surfactant solution (bottom route).

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Liquid marble (LM), a non-stick liquid drop encapsulated by micro-nano particles, offers several advantages such as precise control of evaporation rate, elasticity, robustness, stability and lower friction. They are increasingly used for applications such as gas sensors, micro reactors and platforms for performing chemical/biological reactions. Recent research on LMs have focused on transport and collapsing of liquid marbles, controlled by various external stimuli like electric field, magnetic field, UV light, NIR laser, chemical reaction and acoustic levitation. Though strategies for the controlled transport of LM are well explored, only limited studies are reported on the controlled collapse of the LM over a liquid substrate to release the contained liquid (core-liquid).

Here, we unveil the role of the surface tension of the liquid substrate on the collapse of multilayered LMs and apply this knowledge for realizing a dense planar assembly of microparticles triggered by LM mediated capillary interactions. The results are explained on the basis of the balance between surface tension forces acting on the LM (Fs)and its weight (Fw)  Force analysis reveals that the collapse of the LM on the liquid substrate occurs when the surface tension force approaches to its weight, i.e. when Fs ~ Fw. This has been verified for LMs having volume in the range 6-10 µL. The experiments with different surfactants (an anionic and a cationic) lead to similar results which indicate that the collapse condition of the LMs is mainly dependent on its weight and the surface tension of the liquid substrate. Further, we demonstrate LM mediated assembly of particles at the liquid surface, and interestingly the LM can be collapsed once the assembly is completed, leading to a denser well packed assembled structure. We believe the presented results could provide new insights in the field of microfluidics, particle patterning and assembly.

 

In addition to this, research on fabrication of Janus Liquid marbles is gaining traction these days owing to its better manipulability. However, the existing methods of fabrication are either complex or involve multi-steps. The capabilities of LMs can be further improved by employing multi-particle LMs, commonly referred to as Janus Liquid Marble (JLM). The functionally distinct surface of Janus Liquid marble enables the selective and precise manipulation of liquid marble in a controllable manner. However, the fabrication of JLM remains a challenging task. Herein, we introduce a simple and additive-free JLM fabrication strategy based on the impact of a liquid droplet on a particle bed under a controlled environment and the tunability in particle coverage is achieved by controlling the surface tension of the core liquid.
The working principle of the develped impact-based JLM fabrication technique relies on the particle redistribution on a sparsely coated droplet due to the internal and surface flows created upon its impact on a solid surface at a weber number, We ≥30. The sparsely covered liquid droplet (particle 1) of volume 5-10 μL is allowed to impact on the bed of particles 2 at a weber number =30±1. The surface and internal flows redistribute the particles 1 to the crest of the drop, and the bare liquid surface in contact with the particle 2 bed collects particles from it. Finally, the drop regains its spherical shape resulting in a JLM covered with particle 1 and particle 2.
 

 

 

 

 

 

 

           Figure showing the high speed images showing the impact dynamics of Janus liquid marble formation( Captured at 6200 fps) 

 

Furthur, the impact-driven continuous production of Janus liquid marbles was realized, and the tunability in particle coverage was achieved by controlling the surface tension of the core liquid and volume of the droplet. The impact dynamics of the particle-laden droplet at We = 30 ±1 on both hydrophobic substrate and particles bed were studied. The effect of volume and surface tension of the impacting droplet on JLM particle coverage were analyzed. The value of the maximum spreading parameter (βm) was estimated based on a model developed on energy balance analysis. The experimentally obtained βm was found to be in good agreement with the estimated value.

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Related Publications

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1. Pradip Singha, Sreena Swaminathan, Ajeet Singh Yadav, Subramanyan Namboodiri Varanakkottu* (2019), Surfactant-Mediated Collapse of Liquid Marbles and Directed Assembly of Particles at the Liquid Surface, Langmuir, 35, 4566-76.(*Corresponding author).

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2. B. S. Lekshmi, A. S. Yadav, P. Ranganathan, and S. N. Varanakkottu, ‘Simple and Continuous Fabrication of Janus Liquid Marbles with Tunable Particle Coverage Based on Controlled Droplet Impact’, Langmuir, vol. 36, no. 50, pp. 15396–15402, Dec. 2020.

 

3. B. S. K. Lekshmi and S. N. Varanakkottu, ‘Fabrication of Janus Liquid marbles With Tunable Particle Coverage by Controlling the Surface Tension of the Core Liquid’, IOP Conf. Ser. Mater. Sci. Eng., vol. 1221, no. 1, p. 012026, Mar. 2022.

 

 

Affiliated person

Lekshmi B S