Controlling Liquid Spreading Using Microfabricated Undercut Edges (2013)

A purely topographical method for controlling liquid spreading by using easy-to-fabricate undercut edges is reported. By periodic repetition of such edges, it is shown that multiple droplets can be patterned in well-controlled shapes, and highly anisotropic wetting can also be achieved at a large scale. Apparent contact angles close to 180° at the edge are shown, even for low surface tension liquids. Controlled spreading of liquids deposited on a solid surface has seen an increasing research interest over the past decade. Since the groundbreaking work by Wenzel1 and Cassie,2 topographical and chemical surface structuring has been employed to create a multitude of new engineered surfaces for various applications. These surfaces are often inspired by nature (e.g., the lotus leaf3) and can have wetting properties from superhydrophilic,4 superhydrophobic,5, 6 to superoleophilic and superoleophobic,7-9 and even omniphobic.10-12 Surface structures can also induce anisotropic wetting13, 14 or directional wetting,15, 16 allowing a great degree of control over liquid spreading. On the other hand, inhibiting liquid spreading at one, well-defined boundary is the critical issue in a wide range of applications, from surface-directed liquid flow,17-19 droplet-based microfluidics,20 capillary imbibition,21 to passive filling of fluidic channels by capillary action,22 droplet shape control in biomedical applications,23 microfluidics for biological studies,24 fluidic optics,25 and self-alignment of microchips.26-28 In these applications, liquid overflow at the boundary would be catastrophic. Therefore, it is crucial to maximize the advancing contact angle at the boundary, regardless of whether the liquid is in the Cassie or the Wenzel state. In this paper, we report an easy-to-fabricate, purely topographical structure of undercut edge that can pin the triple contact line (TCL) of liquid on any single edge. By mere periodic repetition of such edges, we show that multiple droplets can be patterned in well-controlled shapes, and highly anisotropic wetting can also be achieved at large scale. The pinning property of the structure also does not depend on surface tension of the liquid. Our microfabrication method for the edges is simple, does not involve chemical surface modification, and consequently can be applied to a wide range of materials, including silicon-based, glass and metals. a) Apparent contact angle of a spreading liquid meeting a sharp edge is constrained by the Gibbs inequality (Equation 1). b) SEM micrograph of a circular pad with undercut edge structure. The top part is thermally grown silicon oxide and the background is silicon. The diameter of the circle is 100 μm, the height of the structure 4 μm, and distance of the undercut 3 μm. c) Optical microscopy image of a hexadecane droplet on a circular pad. The diameter of the pad is 300 μm and diameter of the droplet is approximately 900 μm. d) SEM micrograph of thermally cured adhesive on undercut circular pad. This is known as the Gibbs inequality condition, where θ is the equilibrium contact angle, as measured through the liquid, ϕ is the edge angle through the solid and θ0 is the natural contact angle of the liquid on the surface. It should be noted that in reality, edges are not mathematically sharp. However, roundedness does not affect the upper limit of the apparent contact angle given by Equation 1. For an undercut sharp edge (ϕ ≈ 0°), Equation 1 leads to a theoretical upper bound for equilibrium contact angle of 180° or more, depending on the liquid contact angle on the surface. Such a sharp edge provides ultimate pinning even for liquids that otherwise completely wet the surface (θ0 ≈ 0°). In this paper, we show that a single undercut edge, regardless of θ0, is able to prevent liquid spreading over the edge, while allowing free spreading along the edge. This work is remotely related to the undercut structures previously used to create superoleophobic9 and omniphobic surfaces.10 However, in those works the structures were applied as a matrix of individual pillars, with the droplet spanning over a large number of pillars. Furthermore, the microfabrication was combined with low energy chemical surface treatment, providing a way to induce extreme resistance to wetting of low surface tension liquids. In our work, we employ continuous, closed undercut edges to confine liquids in well-defined shapes, at both micro- and macroscale. Specifically, we demonstrate such confinement capability using a highly wetting silicon and silicon oxide surface without chemical treatment. Therefore, the approach should work with a wide range of materials and is not limited by surface chemistry properties. The undercut structures can easily be realized on a substrate with a distinct top layer material. The top layer is patterned and serves as a mask for undercut etching of the underlying substrate. We used a simple fabrication technique starting from thermally oxidized silicon wafers. Standard photolithography and anisotropic dry etching were used to pattern the oxide, followed by isotropic dry etching of silicon for the undercut. No further processing is needed. We realized the undercut structure in different shapes, starting from a plain circle, as shown in the scanning electron microscope (SEM) micrograph of Figure 1b. To test the pinning capability of the edge, we dispensed a low surface tension liquid (hexadecane, γ = 27 mN m−1) incrementally on the circle to create a growing droplet, until it overflowed. The dispensing was carried out using a pneumatically actuated liquid dispensing pen mounted on a high-accuracy robotic positioning system on a vibration isolation table. An optical microscope image (Figure 1c) shows the apparent contact angle of hexadecane being close to 180° before the droplet collapsed. We repeated the experiment using a thermally curable adhesive (Delo, Monopox AD VE 111986, γ = 34 mN m−1) and stopped the dispensing when the apparent contact angle, judged by optical microscope image, was close to 180°. We then cured the adhesive in a convection oven (30 min at 130 °C), and observed it in SEM to get a closer view of the contact angle. The SEM micrograph (Figure 1d) shows that the contact angle is indeed close to 180°. When the droplet became large enough (typically 1 mm or larger in diameter), it was more likely to collapse and flow over the edge. There are several plausible explanations for this behavior, including a) gravity becoming significant compared to capillary forces (e.g, capillary length of hexadecane is 1.9 mm); b) increased inertia of the droplet under external disturbances leading to contact line failure; c) contamination, such as dust particles, contacting the increased droplet surface area and bridging the liquid to the background. Moreover, regardless of the droplet size, fabrication defects in the edge or careless handling of the sample may break the contact line pinning. Whatever the actual cause, we note that any pinning failure is irreversible and will lead to catastrophic spreading of the liquid on the surface. Nevertheless, reliability of the pinning was still rather high, as all the experiments were conducted in ordinary indoor environment and no ultra high cleanliness was required. To tackle the irreversibility of the pinning failure, we added more edges parallel to the first edge, which provides robustness to all the failure modes discussed above. Periodic repetition of the edges guarantees that liquids maintain their shape even when spreading in the undesired direction. We fabricated patterns consisting of concentric circles and squares with 100 μm and 200 μm pitches to demonstrate this effect (Figure 2a). Water (γ = 72 mN m−1), ethylene glycol (γ = 48 mN m−1) and hexadecane (γ = 27 mN m−1) were dispensed on the patterns. Advancing and receding contact angles were measured both on flat silicon oxide (water: 45°/15°, hexadecane: ≈8°/≈2°) and the background silicon (water: 52°/30°, hexadecane: ≈5°/≈2°). Regardless of the droplet dispensing location, the triple contact line (TCL) spreads only parallel to the edges and the droplet takes the shape of the pattern. In case of a pinning failure, the liquid fills the groove between adjacent edges and is then confined by the next edge. This allows for precise control of droplet shape in two dimensions, regardless of droplet volume and with good robustness to spatially inaccurate dispensing. We also dispensed different liquids on the same concentric circle pattern (Figure 2b). The annular droplets do not mix as long as they are dispensed on different rings and not in excessive amounts. This kind of “complex droplets” have previously been shown by creating chemically modified domains on a nanorough silicon surface.32 Our approach uses purely topographical barriers instead of surface energy contrast to confine the droplets. It should be noted that adding more edges to the same pattern introduces possibility for air to get trapped between the edges, and therefore the droplet may be in Cassie or Wenzel state. This does not affect the observed contact angles at the edges of concentric circles, as a transition to Wenzel state is not accompanied by spreading of the liquid over the pinning edge of the pattern. The shape of the droplets is not due to circular shapes having minimal surface area. To highlight that the droplet takes the shape of the underlying surface pattern, we dispensed hexadecane on a pattern of concentric squares (Figure 2c). The droplet has clearly assumed a square shape. a) SEM micrograph of concentric circles with undercut edges. b) Different liquids (water dyed blue, ethylene glycol dyed yellow, hexadecane dyed red) on the same concentric circle pattern. The outer diameter of the hexadecane circle is 1 cm. c) Hexadecane on concentric square pattern. d) SEM micrograph of parallel stripes of undercut edges. (inset) Close up of the undercut structure. e) ≈10 μl droplets of different liquids spreading on striped structures exhibit significant anisotropic wetting. Hexadecane spread all the way to both edges of the 100 mm wafer. The dyeing is the same as before. f) Water droplet (≈20 μl) on a 90° tilted wafer, view parallel to the edges. Another interesting property of periodically repeating edges is their ability to induce anisotropic wetting. Straight, parallel edges allow wetting along the direction of the edge, and inhibit it in the perpendicular direction. This type of topographical structures for anisotropic wetting has been widely studied in recent research.14, 33-37 Based on the Gibbs inequality, undercut straight edges and highly wetting liquid are expected to exhibit significant anisotropy. To demonstrate this phenomenon, we fabricated a surface of stripes with 100 μm pitch (Figure 2d). Dispensing water, ethylene glycol and hexadecane on the surface revealed extreme anisotropic wetting behavior (Figure 2e). When the liquid was dispensed carefully, the droplet could first be observed in Cassie state, but eventually transitioned irreversibly to Wenzel state and spread fast along the grooves. A faint preceding liquid film spreading in advance of the TCL could be observed (visible in Figure 2e, see also Supporting Information, video 1). The wafer was also tilted up to 90° perpendicularly to the edges to show the inhibited wetting in that direction (Figure 2f). Even with 90° tilt, the water droplet remained pinned and did not move down on the surface. The anisotropic wetting property of periodically repeating undercut edges can also be utilized by creating closed paths of them, forming macroscopic shapes that can be applied e. g., in liquid transportation. In order to test this, a complex pattern was realized on a 100 mm wafer (Figure 3) using concentric paths of the undercut edges, covering the whole wafer surface. A flat, larger area serves both as a reservoir for liquid and as the starting point for the path. We dispensed hexadecane continuously on the starting area until it was clearly pinned and added no liquid afterwards. The liquid started spreading by wetting along the edges. Full wetting of the path was achieved over a time of approximately one hour, without moving the wafer on the flat horizontal surface. Surface microflow of hexadecane on a pattern consisting of closed concentric paths of the undercut edges. The arrows indicate the direction of flow. The wafer diameter is 100 mm. A flat, larger area serves both as a reservoir for liquid and as the starting point for the path. Full wetting of the path took approximately 1 h. The liquid spread without any external actuation after initial dispensing. Inset: Optical microscopy image of the concentric paths. In summary, we have reported a topographical method for inhibiting and controlling liquid spreading on surfaces. We have shown apparent contact angles close to 180° and demonstrated controlling liquid droplet shape, complex droplets, anisotropic wetting, and liquid transportation. The method is based on the pinning effect of undercut edges and is suitable for all partially wetting liquids (θ0 > 0°); the maximum pinning contact angles can reach 180°. Behavior of perfectly wetting liquid/material combinations remains to be tested. The surface is also relatively stable, because no coating is involved, and therefore the surface can easily be cleaned and reused. The method is more sensitive to external disturbances, edge defects (cracks, dents etc.) and impurities (dust, dirt), which can form bridges for the liquid to go over the edge, but we have shown that periodically repeating edges provides robustness to such failures. We also envision complementing the undercut edge effect with nanoroughness or chemical treatment on the background or top material, in order to modify the natural contact angles. Fabrication of the Undercut Structures: A 500 μm-thick silicon (100) wafer with 1100 nm thermal silicon oxide on top was patterned in a standard photolithography process using the AZ5214E photoresist. Oxide not covered by the resist was then anisotropically etched away by reactive ion etching (RIE) in an Oxford Plasmalab 80 etcher, with CHF3/Ar process gases (25/25 sccm), 200 W forward power, and 30 mTorr pressure, for 34 min. After the etching, the photoresist was stripped in an acetone ultrasound bath, followed by isopropyl alcohol rinse. The same RIE etcher was used again to isotropically etch silicon, going under the edges of oxide patterns. The silicon etch was performed in pure SF6 gas (100 sccm), 100 W forward power and 100 mTorr pressure, for 12 min. Supporting Information is available from the Wiley Online Library or from the author. This research was funded by the Graduate School of Electronics, Telecommunication and Automation and the Academy of Finland under Grant 134206. The authors thank Academy Research Fellow, Dr. Robin Ras, for helpful discussions on wetting phenomena and Senior Research Scientist, Dr. Victor Ovchinnikov, for advice regarding fabrication of the microstructures. We also acknowledge the provision of facilities and technical support by Aalto University at Micronova Nanofabrication Centre. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.