Modelling unidirectional liquid spreading on slanted microposts

Cavalli, Andrea; Blow, Matthew L.; Yeomans, J. M.

doi:10.1039/c3sm00043e

Cited by 12 publications

(14 citation statements)

References 15 publications

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“…A total of six samples were fabricated for this study. Following the analysis of previous publications, we approximate the bent nanowires as slanted nanowires as shown in Figure a(iv). Figure b shows some examples of the bent nanowires that were fabricated and the geometrical parameters describing the nanowires for each sample can be found in Table 1 .…”

Section: Resultsmentioning

confidence: 99%

“…The prevalent model used to predict whether unidirectional wetting occurs on a surface or not is based on the premise that wetting in a particular direction depends on the ability of the contact line to reach the next row of nanostructures from the preceding row . Quantitatively, this means that for wetting to occur in the + X direction, for instance, the intrinsic contact angle of the material forming the base (in this case, Au), α Au , must be smaller than ψ + .…”

Section: Resultsmentioning

confidence: 99%

“…In all of the cases studied thus far on structurally asymmetric nanostructures, droplets exhibiting unidirectional wetting characteristics are found to adopt either the hydrophilic Wenzel state or Cassie‐Baxter state . To achieve unidirectional wetting in the hydrophobic Wenzel regime, current theoretical models predict that heavily bent nanostructures are required, and experimental data for borderline cases ( α = 65°) appear to support this hypothesis …”

Section: Introductionmentioning

confidence: 96%

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Unidirectional Wetting in the Hydrophobic Wenzel Regime

Lai

Choi

2015

Adv Materials Inter

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wetting property on desired surfaces. [1][2][3] In addition, it was also found that unidirectional wetting can also be achieved on a surface through the deposition of a coating with a surface energy gradient, [ 10 ] or by decorating it with an orderly array of Janus nanostructures, which possess a hydrophilic face on one side and a hydrophobic face on the other. [ 5,11,12 ] When a water droplet is deposited on these nanostructures and if the substrate material is suffi ciently hydrophilic ( α < 65°, where α refers to the intrinsic water contact angle of the substrate material), the liquid can infi ltrate into the space between the nanostructures, wetting their sides and the base. [ 1 ] The droplet then exhibits a net contact angle that is less than 90°. We shall refer to this case as the hydrophilic Wenzel state. However, if the substrate material is not as hydrophilic (65° < α < 90°) and the water droplet still wets the nanostructures fully, the droplet will most likely exhibit a net contact angle that is greater than 90° due to strong contact line pinning at the edges of the nanostructures. [ 13 ] This shall be referred to as the hydrophobic Wenzel state. Finally, if the substrate material is intrinsically hydrophobic ( α > 90°), then the liquid droplets would adopt the Cassie-Baxter state instead, wetting only the tips of the nanostructures, effectively sitting on a fl at, composite surface made up of air and nanostructure tips. [ 2,14 ] In all of the cases studied thus far on structurally asymmetric nanostructures, droplets exhibiting unidirectional wetting characteristics are found to adopt either the hydrophilic Wenzel state [ 1,3,5,15 ] or Cassie-Baxter state. [ 2 ] To achieve unidirectional wetting in the hydrophobic Wenzel regime, current theoretical models predict that heavily bent nanostructures are required, [ 1,15,16 ] and experimental data for borderline cases ( α = 65°) appear to support this hypothesis. [ 1,15 ] In this report, however, we show that, in contrast to these expectations, unidirectional wetting in the hydrophobic Wenzel regime can be induced by slightly bent nanostructures. The hydrophobic Wenzel state was achieved by allowing water droplets to spread on an orderly array of bent hydrophilic Si nanowires protruding from a hydrophobic base of Au coating, an arrangement that is made possible by the use of oblique angle deposition and metal assisted chemical etching of Si. Furthermore, through the novel use of simple NaCl precipitation, we were able to observe the nanoscale features of the droplet near the triple phase contact line, obtaining important experimental insights into the mechanism of unidirectional wetting on asymmetric nanostructures.Unidirectional wetting surfaces can cause liquid droplets to fl ow/move in one direction while pinning them in the other directions, a feature that is useful for biosensing, adhesives, thermal management, and microfl uidics. Such surfaces can be fabricated by employing structurally or chemically asymmetric nanostructures. While unidirectional wet...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

See 1 more Smart Citation

Unidirectional Wetting in the Hydrophobic Wenzel Regime

Lai

Choi

2015

Adv Materials Inter

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“…On the entire series of TPAs (micro-and nanoscale), a deionized water droplet spreads preferentially in the direction of the pillar tilt, with spreading ratios ranging from 1.5:1 to 2.5:1, as shown in Table 1. 39,43,44 On the TRAs, a deionized water droplet sits symmetrically on top of the ridges (initial θ* = 110°) and spreads preferentially down the valleys between the ridges. The final apparent contact angle (θ*) of an irreversibly impaled water droplet on all length scales of TPAs was θ* < 10°, indicating that the surfaces were superhydrophilic at room temperature.…”

Section: Resultsmentioning

confidence: 99%

Length scale of Leidenfrost ratchet switches droplet directionality

et al. 2014

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Arrays of tilted pillars with characteristic heights spanning from hundreds of nanometers to tens of micrometers were created using wafer level processing and used as Leidenfrost ratchets to control droplet directionality. Dynamic Leidenfrost droplets on the ratchets with nanoscale features were found to move in the direction of the pillar tilt while the opposite directionality was observed on the microscale ratchets. This remarkable switch in the droplet directionality can be explained by varying contributions from the two distinct mechanisms controlling droplet motion on Leidenfrost ratchets with nanoscale and microscale features. In particular, asymmetric wettability of dynamic Leidenfrost droplets upon initial impact appears to be the dominant mechanism determining their directionality on tilted nanoscale pillar arrays. By contrast, asymmetric wetting does not provide a strong enough driving force compared to the forces induced by asymmetric vapour flow on arrays of much taller tilted microscale pillars. Furthermore, asymmetric wetting plays a role only in the dynamic Leidenfrost regime, for instance when droplets repeatedly jump after their initial impact. The point of crossover between the two mechanisms coincides with the pillar heights comparable to the values of the thinnest vapor layers still capable of cushioning Leidenfrost droplets upon their initial impact. The proposed model of the length scale dependent interplay between the two mechanisms points to the previously unexplored ability to bias movement of dynamic Leidenfrost droplets and even switch their directionality.

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“…This approach has proved quite effective to delineate the wetting phase diagram as a function of inclination angle 17 . Precise numerical investigations however demonstrate that the result holds only for relatively hydrophilic surfaces 20 . Experimentally, it was also found to fail for denser structures 16 .…”

Section: Introductionmentioning

confidence: 96%

Wetting against the nap – how asperity inclination determines unidirectional spreading

2016

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We have carried out wetting experiments on textured surfaces with high aspect ratio asperities in the Wenzel state. When inclination is imparted to the asperities, we observe a strictly unidirectional spreading opposite to the direction in which the asperities point. The advancing contact angle decreases markedly as inclination increases. A crude numerical analysis successfully accounts for this behaviour, highlighting the interplay between Gibbs pinning at the top of the structures and imbibition along the valleys between them. In Gibbs pinning non-linearities play a major role and we find that simple line averaging - i.e. a rule of mixture - cannot account for this evolution except for weak surface perturbations, i.e. large inclinations.

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Modelling unidirectional liquid spreading on slanted microposts

Cited by 12 publications

References 15 publications

Unidirectional Wetting in the Hydrophobic Wenzel Regime

Unidirectional Wetting in the Hydrophobic Wenzel Regime

Length scale of Leidenfrost ratchet switches droplet directionality

Wetting against the nap – how asperity inclination determines unidirectional spreading

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