Uni-directional liquid spreading on asymmetric nanostructured surfaces

Journal name:
Nature Materials
Volume:
9,
Pages:
413–417
Year published:
DOI:
doi:10.1038/nmat2726
Received
Accepted
Published online

Controlling surface wettability and liquid spreading on patterned surfaces is of significant interest for a broad range of applications, including DNA microarrays, digital lab-on-a-chip, anti-fogging and fog-harvesting, inkjet printing and thin-film lubrication1, 2, 3, 4, 5, 6, 7, 8. Advancements in surface engineering, with the fabrication of various micro/nanoscale topographic features9, 10, 11, 12, 13, and selective chemical patterning on surfaces14, 15, have enhanced surface wettability3, 16, 17 and enabled control of the liquid film thickness18 and final wetted shape19. In addition, groove geometries and patterned surface chemistries have produced anisotropic wetting, where contact-angle variations in different directions resulted in elongated droplet shapes20, 21, 22, 23, 24, 25, 26. In all of these studies, however, the wetting behaviour preserves left–right symmetry. Here, we demonstrate that we can harness the design of asymmetric nanostructured surfaces to achieve uni-directional liquid spreading, where the liquid propagates in a single preferred direction and pins in all others. Through experiments and modelling, we determined that the spreading characteristic is dependent on the degree of nanostructure asymmetry, the height-to-spacing ratio of the nanostructures and the intrinsic contact angle. The theory, based on an energy argument, provides excellent agreement with experimental data. The insights gained from this work offer new opportunities to tailor advanced nanostructures to achieve active control of complex flow patterns and wetting on demand.

At a glance

Figures

  1. Comparison of wetting behaviour on symmetric and asymmetric nanostructured surfaces.
    Figure 1: Comparison of wetting behaviour on symmetric and asymmetric nanostructured surfaces.

    a, Axially symmetric liquid spreading of a 1μl droplet of deionized water with 0.002% by volume of surfactants (Triton X-100) deposited on typical vertical nanopillars with diameters of 500nm, spacings of 3.5μm and heights of 10μm (inset). b, Uni-directional liquid spreading of a droplet on the same dimension nanostructures as a, but with a 12° deflection angle (inset). The images show the characteristics of a spreading droplet at one instant in time. The scale bars in the insets are 10μm.

  2. Scanning electron micrographs with uniform arrays of asymmetric nanostructured surfaces.
    Figure 2: Scanning electron micrographs with uniform arrays of asymmetric nanostructured surfaces.

    The nanostructure deflection angles, ϕ, are 7°, 12°, 25° and 52°. The diameters, spacings and heights of the nanopillars are 500–750nm, 3.5μm and 10μm, respectively. The Cartesian coordinate system is defined such that the pillars deflect in the positive X (+X) direction. Although each surface has the same equilibrium contact angle (4°–8°, depending on the thickness of the gold film), the spreading behaviour is dependent on the deflection angle. Inset: Schematic defining the deflection angle.

  3. Time-lapse images of uni-directional spreading of a liquid droplet.
    Figure 3: Time-lapse images of uni-directional spreading of a liquid droplet.

    a,b, Side view (a) and top view (b) of a 1μl droplet of deionized water with 0.002% by volume of surfactant spreading on a surface with pillar diameters of 500nm, spacings of 3.5μm and heights of 10μm (with a height-to-spacing ratio of 2.87, H/l=2.87) deflected at 12° as shown in Fig. 2. The initial location of the droplet contact line in the −X direction is indicated by the dashed lines, where the contact line stays pinned throughout the spreading process. A liquid film propagates ahead of the macroscopic droplet as highlighted by the arrow in b.

  4. Experimental results and the theoretical curves predicting uni-directional liquid spreading.
    Figure 4: Experimental results and the theoretical curves predicting uni-directional liquid spreading.

    The fabricated asymmetric pillars have diameters ranging from 500 to 750nm, spacings of 3.5μm and heights of 10μm. The squares, circles, triangles and crosses show experimental results of uni-directional, bi-directional, nearly uni-directional and no liquid propagation, respectively. The colours of these symbols indicate the different liquids used in the experiments. The theoretical curves based on the proposed model are shown for θeq=θcr with varying deflection angles, ϕ, for H/l=2.87. The black solid and dotted curves correspond to the critical contact angles, θcr,+X and θcr,−X. The white dotted line represents the condition for imbibition where θeq=65°, which is obtained by experiments. The centre region (blue) bound by θcr,−X and θeq=65° represents the parameter space that leads to uni-directional liquid spreading. Inset: Schematic diagrams explaining the geometries for the proposed model to determine the critical angle, θcr, in the −X (left) and +X (right) directions.

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Affiliations

  1. Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave. 3-461B, Cambridge, Massachusetts 02139, USA

    • Kuang-Han Chu,
    • Rong Xiao &
    • Evelyn N. Wang

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All authors contributed to designing and conducting the experiments, model development and preparing the manuscript.

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The authors declare no competing financial interests.

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