Superhydrophobic Tracks for Low‐Friction, Guided Transport of Water Droplets

Mertaniemi, Henrikki; Jokinen, Ville; Sainiemi, Lauri; Franssila, Sami; Marmur, Abraham; Ikkala, Olli; Ras, Robin H. A.

doi:10.1002/adma.201100461

Cited by 206 publications

(157 citation statements)

References 22 publications

(27 reference statements)

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“…The slip can reduce viscous drag 16,17 and thus enhance flow in micro-and nanochannels 15 as well as in sessile droplets [19][20][21] . Combining the reduced drag with the guidance of the droplets by gravitational 18 , electric 18 or magnetic fields 19,20 opens platforms for novel functional droplet devices performing, for example, logic operations or programmable chemistry 21 .…”

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confidence: 99%

Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

et al. 2013

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The recently demonstrated extremely water-repellent surfaces with contact angles close to 180°with nearly zero hysteresis approach the fundamental limit of non-wetting. The measurement of the small but non-zero energy dissipation of a droplet moving on such a surface is not feasible with the contemporary methods, although it would be needed for optimized technological applications related to dirt repellency, microfluidics and functional surfaces. Here we show that magnetically controlled freely decaying and resonant oscillations of water droplets doped with superparamagnetic nanoparticles allow quantification of the energy dissipation as a function of normal force. Two dissipative forces are identified at a precision of B10 nN, one related to contact angle hysteresis near the three-phase contact line and the other to viscous dissipation near the droplet-solid interface. The method is adaptable to common optical goniometers and facilitates systematic and quantitative investigations of dynamical superhydrophobicity, defects and inhomogeneities on extremely superhydrophobic surfaces.

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Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

et al. 2013

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“…Minute amounts of proteins or biomolecules can be deposited on arrays of micropillars, offering promising routes to sensitive high-throughput analysis on laboratory-on-chip benches (18). Patterned superhydrophobic coatings provide new strategies to guide drops (19) or to tune drop adhesion (20). Nanocrystals can be deposited (21) and grown (22) on the tips of a micropillar arrays.…”

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confidence: 99%

How superhydrophobicity breaks down

Papadopoulos

Mammen

Deng

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

432

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A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar's circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water-air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.micropillars | pinning | wetting transition | contact angle | lithography | self-cleaning | hysteresis

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“…54 Self-assembly was performed using the air/water interface as a template following a protocol from literature. 37 In brief, the colloidal dispersion was diluted with ethanol (50% by volume) and added to the air/water interface via a hydrophilic glass slide immersed into the water subphase at an angle of approximately 45 . At the three phase contact line, the colloids assemble into a close-packed monolayer.…”

Section: Colloidal Synthesis and Assembly Into Monolayersmentioning

confidence: 99%

“…of research areas, including multiplexed cell culture and cell isolation without cross-contamination, 42,43 microuidic chips with controlled water transport following predened pathways, [44][45][46] highly-sensitive detection of analytes, 44,47,48 capillarydriven self-alignment of microscopic objects, 49 dynamic control of patterning, 50 or chemical synthesis in connement; for example to fabricate metal-organic framework microsheets with tailored dimensions. 51 However, the spatial connement of lowsurface-tension liquids such as organic solvents is substantially more difficult because of their highly wetting behavior that has been shown so far to be prevented only by creating quite complex, lithographically dened surface structures.…”

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Tailoring re-entrant geometry in inverse colloidal monolayers to control surface wettability

et al. 2016

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Controlling the microscopic wetting state of a liquid in contact with a structured surface is the basis for the design of liquid repellent as well as anti-fogging coatings by preventing or enabling a given liquid to infiltrate the surface structures. Similarly, a liquid can be confined to designated surface areas by locally controlling the wetting state, with applications ranging from liquid transport on a surface to creating tailored microenvironments for cell culture or chemical synthesis. The control of the wetting of a low-surfacetension liquid is substantially more difficult compared to water and requires surface structures with overhanging features, known as re-entrant geometries. Here, we use colloidal self-assembly and templating to create two-dimensional nanopore arrays with tailored re-entrant geometry. These pore arrays, termed inverse monolayers, are prepared by backfilling a sacrificial colloidal monolayer with a silica sol-gel precursor material. Varying the precursor concentration enables us to control the degree to which the colloids are embedded into the silica matrix. Upon calcination, nanopores with different opening angles result. The pore opening angle directly correlates with the re-entrant curvature of the surface nanostructures and can be used to control the macroscopic wetting behavior of a liquid sitting on the surface structures. We characterize the wetting of various liquids by static and dynamic contact angles and find correlation between the experimental results and theoretical predictions of the wetting state based on simple geometric considerations. We demonstrate the creation of omniphobic surface coatings that support Cassie-Baxter wetting states for liquids with low surface tensions, including octane (g ¼ 21.7 mN m À1). We further use photolithography to spatially confine such low-surface-tension liquids to desired areas of the substrate with high accuracy.

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Superhydrophobic Tracks for Low‐Friction, Guided Transport of Water Droplets

Cited by 206 publications

References 22 publications

Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

How superhydrophobicity breaks down

Tailoring re-entrant geometry in inverse colloidal monolayers to control surface wettability

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