STIMULI RESPONSIVE MICROUIDICS

 

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Open surface digital microfluidics (OSDM) is an emerging technology where the microfluidic transport phenomena as well as chemical or biological analysis can be performed over open surfaces. OSDM offers added advantages such as less volume requirement and programmability over closed microfluidic channels.   Major concern with the solid surfaces is the large hysteresis contact angle while handling water droplets, which arises due to the chemical or physical heterogeneities of the surface. As a result, transport over the solid surfaces is limited to specific type of liquids only.. Remote control over the wettability of such a surface is an added advantage for developing efficient lab on chip devices, which can be achieved by controlling surface topography or surface chemistry by applying external stimuli.

In this context, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a structured polymer surface using temperature-induced wettability gradient. This is achieved by grafting a layer of thermoresponsive polymer calledpoly(N-isopropylacrylamide)(PNIPAAm). Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this “transport zone” wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s.

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Optically controlled droplet transport platform

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Precise control over the transport and assembly of tiny liquid droplets using external stimuli is an emerging field of physics. Conventional methods for the transport of liquid droplets on solid surfaces are based on surface tension gradient, electrowetting/optoelectrowetting and magnetic, electrostatic and gravitational forces etc. Since the transport depends on the properties of droplet or the substrate for the conventional methods, there are constraints of the droplets which can be transported for those methods. But the method proposed in our work is advantageous where the transport does not rely on the chemical properties of both liquid droplet and substrate.

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                                                      Optically Controlled Liquid Marble Transport Platform

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On-demand transport of Liquid marbles (LMs) over solid/liquid substrate is of particular interest in view of their potential applications in digital microfluidics. Transport methods based on optical actuation are promising as it ensures additive-free and contactless manipulation. Moreover, it does not rely on the properties of the core liquids or substrate. Herein, we demonstrate an on-demand liquid marble transport platform based on the local surface deformation of a polymer layer controlled by the thermal expansion of the confined air cavity. The method relies on the optothermal expansion of the air cavity confined in a Polymethyl methacrylate (PMMA) substrate. The platform is made of acrylic with a 5 mm hole, and a thin super hydrophobic polymer layer over it. The localized heating of the air cavity resulted in the upward deformation (convex deformation) of the PDMS layer, and the localized cooling resulted in the downward deformation (concave deformation). Further, we exploit the upward deformation to realize liquid marble transport over the PDMS layer. The hydrophobicity of the polymer layer is enhanced by coating the surface with Polyvinylidene difluoride (PVDF) nanoparticles for efficient liquid marble transport. We demonstrate the transport of liquid marble of volume 3 μL with a laser source (450 nm). A localized temperature rise of about 0.6 0C at the surface is sufficient to realize the transport of liquid marbles. The demonstrated strategy is simple and could be used for the on-demand transport of liquid marble, irrespective of the properties of the liquid.

 

Related Publications

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1.Theneyur Narayanaswamy Banuprasad, Thamarasseril Vijayan Vinay, C. K. Subash, Soney Varghese, Sajan Daniel George, Subramanyan Namboodiri Varanakkottu* (2017), Fast transport of water droplets over a thermoswitchable surface using rewritable wettability gradient, ACS Applied Materials and Interfaces, 9, 28046-28054. http://pubs.acs.org/doi/10.1021/acsami.7b07451 (*Corresponding author). 

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2. C. M. Ragisha and S. N. Varanakkottu, ‘Optically Controlled Liquid Marble Transport Platform’, IOP Conf. Ser. Mater. Sci. Eng., vol. 1221, no. 1, p. 012030, Mar. 2022, doi: 10.1088/1757-899X/1221/1/012030.

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Affiliated Persons

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Vinay T V

Ragisha C M

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