Time-Dependent Liquid Transport on a Biomimetic Topological Surface Cunlong Yu,†,§,⊥ Chuxin Li,†,⊥ Can Gao,† Zhichao Dong,*,† Lei Wu,*,‡ and Lei Jiang†,§ †
CAS Key Laboratory of Bio-inspired Materials and Interfacial Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China ‡ CAS Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China § Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, 100191, People’s Republic of China S Supporting Information *
ABSTRACT: Liquid drops impacting on a solid surface is a familiar phenomenon. On rainy days, it is quite important for leaves to drain off impacting raindrops. Water can bounce off or flow down a water-repellent leaf easily, but with difficulty on a hydrophilic leaf. Here, we show an interesting phenomenon in which impacting drops on the hydrophilic pitcher rim of Nepenthes alata can spread outward to prohibit water filling the pitcher tank. We mimic the peristome surface through a designed 3D printing and replicating way and report a time-dependently switchable liquid transport based on biomimetic topological structures, where surface curvature can work synergistically with the surface microtextures to manipulate the switchable spreading performance. Motived by this strange behavior, we construct a large-scaled peristome-mimetic surface in a 3D profile, demonstrating the ability to reduce the need to mop or to squeegee drops that form during the drop impacting process on pipes or other curved surfaces in food processing, moisture transfer, heat management, etc. KEYWORDS: biomimetic surface, impact, overflow, switchable spreading, unidirectional transportation, superhydrophilic, time dependent
L
asymmetrically structured superhydrophilic surfaces, not to mention the time-dependently switchable impacting−spreading dynamics. The tropical carnivorous pitcher plant, Nepenthes alata, is renowned for digesting arthropod prey to survive in nutrientpoor habitats.22,23 Insects are captured when they “aquaplane” on the peristome surfaces into the conspicuous pitfall traps.24,25 Recently, it has been shown that the surface morphology of the peristome plays a determining role in the outcome of liquid spreading process, where, assisted by capillary rise behavior, deposited water can unidirectionally transport along the peristome surface.26,27 The formation of a water film on the pitcher rim facilitates the slipping of insects into the pitcher.28 The pitcher tank is therefore of great importance to the plant. If the tank is fully filled with water on rainy days, not only does the digestive fluid become dilute, but also the pitcher could break off from the leaf. The lip is thus believed to play a crucial role in providing a shelter for the pitcher from the rain, but with less rain coverage than the pitcher size in an open state.29−32
iquid impact on solid surfaces is a familiar phenomenon in nature and central to many technical applications, including inkjet printing, painting, coating, self-cleaning, and crop spraying.1,2 Depending on the surface textures and surface wettabilities, drop impinging processes exhibit various phenomena including deposition, bouncing, and splashing.3−8 Extensive progress has been made in understanding and controlling drop dynamics on various textured surfaces, driven by the intrinsic scientific interest and practical importance.9−15 Recent advances in the ability to record the impact dynamics and to fabricate micro- or nanostructured surfaces have accelerated the research progress.14−17 Particularly because of the need for nonwetting, self-cleaning, and anti-icing surfaces, the majority of research has focused on the impacting dynamics of drops on superhydrophobic substrates.6,14−18 Drop impact dynamics on macrotextured or curved superhydrophobic surfaces have been recently demonstrated.19−21 Besides superhydrophobic surfaces, there is also a broad palette of superhydrophilic surfaces in nature that are of great significance. Studies have been reported with respect to dynamic wetting on (super)hydrophilic surfaces; however, little work has addressed the role of surface curvatures at different scale in influencing the physics of the drop collision process on © XXXX American Chemical Society
Received: March 9, 2018 Accepted: April 30, 2018
A
DOI: 10.1021/acsnano.8b01800 ACS Nano XXXX, XXX, XXX−XXX
Article
www.acsnano.org
Cite This: ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 1. In situ optical observation of time-dependent water spreading dynamics on the peristome surface of Nepenthes alata. (a) Optical image of a pitcher of Nepenthes alata, showing a conspicuous prey-trapping peristome. (b) Cross-sectional image of the peristome. The peristome is arch-shaped, and the radius of curvature, R1, is approximately 5 mm. (c) Inner scanning electron microscope view of the peristome surface. Microridges are between neighboring microgrooves with an R2 of ∼100 μm. The R3 of the microgroove is ∼500 μm. (d) High-magnification scanning electron microscope image of patterned duck-billed microcavities that exist in the microgroove. (e) Crosssectional view of the shaped edge of the overhang of the microcavity. R4 is in sub-micrometer scale. (f−h) Time sequence images of the timedependent spreading process of a water droplet (D = 1.5 mm) on the peristome surface at a Weber number of 8.2. The three rows are top view, inner-side view, and outer-side view of the peristome surface, respectively. Inertia dominates during the initial impacting process, where water tends to overflow the edge with a much larger RoC. After spreading to its maximum extension in the blue-arrow bidirections at about 2.6 ms, the drop turns the spreading trend to the outer side unidirectionally. After the inertia−capillary time, τ = 2.6 ms, the capillary rise effect acts on the water. The contact lines are pinned at three directions steadily, until the drop fully spreads on the peristome surface in an elongated shape at a time of ∼90 ms.
RESULTS AND DISCUSSION The peristome surface is a typical physical model (Figure 1a), which contains surface curvatures at different scales.28,33 Figure 1a−e show optical and scanning electron microscopic (SEM) images of the peristome surface. To the naked eye the peristome appears as an arch-shaped ring of tissue with a radius of curvature (RoC) of about R1 = 5 mm in a vertical crosssectional view, much larger than that, ∼1 mm, of a water drop (Figure 1b).22 Switching the optical view to a scanning electron micrograph view, the surface of the peristome is covered by a two-order hierarchy of parallel microgrooves (Figure 1c).28 The larger microgroove has an R3 of 500 μm. Between the neighboring microgrooves is the microridge. The RoC of the microridge with an R2 of ∼100 μm is almost one-tenth of the diameter of a water drop. A high-magnification SEM image demonstrates overlapped microcavities form smaller microgrooves (Figure 1d). The microcavity slants into the surface, is enclosed at the tip, and has a sharp overhang edge at the rear side.22 The R4 of the overhanging edge is less than 1 micrometer (Figure 1e). An enclosed microcavity exists below the overhang edge, which acts as a “cage”. If the impacting water does not fill the “cage” in time, air could be stored in the microcavities. The air entrapment resists the spreading of the precursor toward the tip of the microcavity. As shown in Figure 1f−g, in contrast to previous reported
Here, we show that impact drops on the pitcher rim can spread outward to prohibit the water from filling the tank. In contrast to an impact drop retaining a circular symmetry on a flat surface, a drop exhibits time-dependently asymmetric spreading dynamics on the biomimetic topological peristome surfaces. After the impact process, the water drop shows distinct spreading abilities along two perpendicular directions, in which water bidirectionally spreads on peristome surface along the axial direction at the earliest impact time, followed by the unidirectionally spreading on the peristome surface. Using a commercial 3D printer, we can fabricate delicate 3D printing structures through shrinking the replica. By rationally tuning of the surface curvatures at different scales through computeraided design, the curvatures can work collaboratively with inertia and capillary force to significantly adjust the spreading dynamics at different time scales. In general, this work reveals that the overlooked effect of different scaled surface curvatures can play an important role in setting the liquid spreading performance on superhydrophilic substrates with diverse applications, where liquid directional transport control and raindrop shielding abilities are demonstrated as proof-ofconcept designs. B
DOI: 10.1021/acsnano.8b01800 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 2. Fabrication of the peristome-mimetic surface. (a) Schematic demonstration of the fabrication process of a peristome-mimetic poly(vinyl alcohol) (PVA) gel surface. (b) Shrinking process of the PVA hydrogel during the ethanol displacement. A time-lapse video is shown in Movie S1. (c and d) Stereoscopic microscope images of the artificial surface. There is a microgroove distribution with a periodicity similar to the natural Nepenthes peristome, and the microgrooves consist of periodic microcavities. (e, f) SEM images of the peristomemimetic film at the top, cross-sectional views. The curvature changes as the outline of the peristome-mimetic surface varies from microridges with an R5 of 500 μm in between neighboring arrayed microcavities to sharp overhangs with an R6 of 10 μm along the microcavities.
to overflow the ridges in a bidirectional way. During 1.5−2.6 ms, inertia and capillarity both act on the spreading dynamics. After τ (Figure 1h), the dynamic spreading transits from bidirectionally along the axial direction to unidirectionally toward the outside of the peristome along the azimuthal direction. The unidirectional spreading after τ can be easily explained by the act of a capillary effect,26 in which the capillary rise effect directs the motion of the depositing liquid along the overlapped Taylor corner.35 To explore the mechanism of such a switchable “one side pinning and one side driving” water deposition process, we fabricate a peristome-mimetic surface and mount it on the flat substrate and curved surfaces with varying diameters of curvature D between 10 and 40 mm. Control tests are also performed by designing microgroove-structured hydrophilic surfaces and smooth hydrophilic surfaces. The fabrication process of the peristome-mimetic surface is shown in Figure 2a. The mold is constructed through 3D printing. After mounting the 3D printed mold in a tank, we, then, cast a hot poly(vinyl alcohol) (PVA) solution, 70 °C, onto the mold (step 1) and then store the PVA-coated mold in a refrigerator for 6 h (step 1−2). We subsequently remove the PVA replica from the mold (step 2) and submerge the peristome-mimetic flexible PVA replica into water three times every 2 h. The PVA hydrogel is finally prepared and shown in the left image of Figure 2b1 and Figure S1. Considering the resolution limitation of current commercial 3D printing techniques, we, here, provide an alternative method to enhance the resolution and reduce the pattern size by shrinking the dimension of the PVA film through an ethanol displacement method. After 1 h, the pattern decreases nearly a quarter size. The dehydration dynamics is demonstrated in Figure 2b2, Figure S2, and Movie S1a. The PVA gel shrinks a little when increasing the dehydration time (Figure 2b2−b5). This is an advantage for biomimetic researchers to use a desktop 3D printer in acquiring delicate surface morphologies.
unidirectional liquid deposition behavior on the peristome surface,26,28 intriguing impacting and spreading phenomena occur after a high-speed impact. Figure 1f−h present selected snapshots of a drop of diameter D0 = 1.5 mm impinging on the peristome surface with a radius of curvature of ∼7 mm from the top, inner, and outer views, respectively. The impact velocity, v, is 0.89 m s −1 , corresponding to a Weber number, We = ρv2R/γ, of 8.2, and Ohnesorge number, Oh = μ/(ρRγ)1/2, of 2.8 × 10−3, respectively, where R is the drop radius, ρ is the water density, and μ is the water viscosity. The spreading dynamics of impacting drops varies from bidirectional to unidirectional as the spreading time persists. As shown in Figure 1f, when the drop contacts the curved peristome surface, it initially spreads along the axial direction bidirectionally from 0 to 1.5 ms, where t = 0 indicates the moment of drop−substrate contact. As time persists, an intriguing phenomenon is observed after the spreading of water in the axial (straight) direction has stopped spreading at 2.6 ms, in which water in the azimuthal (curved) direction comes to spread toward the outside unidirectionally. As time sequence images show in Figure 1g and h, water is pinned from the inner view (Figure 1g) and spreads continuously from the outer view (Figure 1h). As recording time persists, the drop shrinks in height with an elongated spreading area (Figure 1h). Finally, water maintains an elongated shape along the azimuthal direction at 90 ms on the peristome surface. In the experiment, there are threshold time values for the transition of spreading dynamics from unidirection to bidirections on multiple scaled surface curvatures. The capillary time, τ ≈ (ρR3/γ)1/2, signifies the balancing time between the inertia and capillarity, which is 2.6 ms in this experiment. Considering 1.5 ms is less than τ, the impacting drop dynamics is inertially driven during the initial impact. As the sharp edge could reduce overflow behavior at the overhang when inertial fluid impacts on this kind of surface,1,34 impacting water tends C
DOI: 10.1021/acsnano.8b01800 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 3. Time-dependent water spreading dynamics on a peristome-mimetic surface. (a) Schematic of the experimental setup. A micropump is used to dispense water drops. The peristome-mimetic surface is horizontally mounted on the plate below the nozzle with three high-speed cameras capturing the water impacting and spreading dynamics from top, front, and side views. The red arrow represents the top view, the green arrow represents the front view, and the blue arrow represents the side view. (b) The variation of the ratio between the spreading length, L, and the spreading width, W, (defined as L/W), as a function of spreading time. Water spreads bidirectionally across the ridges and quickly reaches the minimum L/W of 1/1.4 just after the impact. After the inertia−capillary time, the L/W reaches its maximum value of 2.8. (c and d) Time-lapse images from front and side views and corresponding schematic diagrams of the drop spreading process. (e) Within 1.2 ms, water pinning at both front and rear shaped overhangs with air entrapped at the front microcavity. In the right cartoon, Pim is the impact dynamic pressure acting on the air−water interface, Pair is the air pressure inside the microcavities, Fc is the capillary force, and f is the friction resistance. The air cushion inside the microcavity resists the water penetration, and the sharp overhang resists the water’s forward motion. The contact line pinning at the shaped edge leads to the bidirectional spreading of water across the ridges within the inertia−capillary time. (f) Assisted by the capillary rise behavior, water fills the microcavities one-by-one, and the water pattern holds the same width during the capillary rise dominated unidirectional spreading process after the inertia−capillary time.
procedure involves releasing a 2.5 μL water drop, D = 1.7 mm, onto a peristome-mimetic surface and filming the impacting and spreading dynamics with high-speed cameras from top, front, and side views, respectively. To vary the We number, we change the release height of the droplet. Controlling the freefalling height from 0 to 80 mm, the impacting velocity ranges from 0 to 1.25 m s−1, corresponding to We numbers ranging from 0 to 18.8 (Figure S4). Similar impacting and spreading behavior is observed on biomimetic surfaces. As shown in Figure 3, bidirectional transport is obviously observed at the very beginning of the high-speed drop impact (We = 9.4), while unidirectional transport is observed after a duration. A typical way to convey drop impact dynamics is to plot the dimension of the width, W, and length, L, of the drop on the peristome-mimetic surface as a function of time (Figure 3b). The ratios between L and W are recorded by a high-speed camera and analyzed by data analysis
Stereoscopic and scanning electron microscopes are also used to demonstrate the surface morphologies of biomimetic surfaces in detail (Figure 2c−f). Similar to the peristome surface, the biomimetic surface has arrayed microcavities. Microcavities slant into the surface with an angle of 25° (Figure 2e1). Surface curvatures vary as the outline of the microcavity changes from the neighboring microridges, R5 ≈ 500 μm, to the sharp overhangs, R6 ≈ 10 μm (Figure 2e2), along the cavity (Figure 2f). Surface wettability is measured by a contact angle machine. The contact angle of the as-prepared PVA film is approaching 0°, like that for the natural peristome surface (
