Liquids Unidirectional Transport on DualScale Arrays Yifan Si,† Ting Wang,‡ Chuxin Li,‡ Cunlong Yu,† Ning Li,† Can Gao,† Zhichao Dong,*,‡ and Lei Jiang†,‡ ACS Nano Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 07/04/18. For personal use only.
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Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Advanced Innovation Center for Biomedical Engineering, School of Chemistry, Beihang University, Beijing 100191, China ‡ CAS Key Laboratory of Bio-inspired Materials and Interfacial Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Liquids unidirectional transport has cutting-edge applications ranging from fog collection, oil−water separation, to microfluidic devices. Despite extensive progresses, existing man-made surfaces with asymmetric wettability or micro/nanoscales structures are still limited by complex fabrication techniques or obscure essential transport mechanisms to achieve unidirectional transport with both high speeds and large volumes. Here, we demonstrate the three-dimensional printed micro/macro dual-scale arrays for rapid, spontaneous, and continuous unidirectional transport. We reveal the essential directional transport mechanism via a Laplace pressure driven theory. The relationship between liquid unidirectional transport and surface morphology parameter is systematically explored. Threshold values to achieve unidirectional transport are determined. Significantly, dual-scale arrays even facilitate liquid’s uphill running, microfluidics patterning, and liquid shunting in target directions without external energy input. Free combination of dual-scale island arrays modules, just like LEGO bricks, achieves fast liquid transport on demand. This dual-scale island array can be used to build smart laboratory-on-a-chip devices, printable microfluidic integration systems, and advanced biochemistry microreactors. KEYWORDS: unidirectional transport, dual-scale, 3D printing, microfluidics devices, Laplace pressure difference
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patterns to obtain unbalanced surface tension forces or complex topological structures.3,14,17−23 Sophisticated, highcost, and time-consuming fabrication methods are therefore unavoidably applied to construct these substrates. Furthermore, researchers often need to combine advanced instrument and equipment to deduce or suppose essential directional transport mechanism, which limits their practical applications.14,15,18,24 Additionally, fine micronanoscale structures are fragile under external force, which produces great limitations on the service life of the substrate. Considering these factors, as a hypothesis, macroscopic structured surface would be an alternative choice for unidirectional transport devices with great developments in the future of this research field. Here, we demonstrate a liquid unidirectional transport manipulation mechanism via open micro/macro dual-scale arrays to achieve long-range, rapid, and controlled transport with the act of overlapped Laplace pressure as the driving force. Liquid with low surface tension can transport
irectional transport of liquid on artificial solid surfaces without external force is a lively research field for its tremendous application potential range from fog collection, agricultural irrigation, lubrication, oil−water separation, to microfluidic operation.1−6 Dramatically, lots of living organisms in the biosphere, such as back of desert beetles, spider silk, cactus spines, bird beaks, lizard skin, and the peristome of pitcher plants, possess the ability of liquid transport with immaculate precision by taking advantage of their microscaled morphologies or wettabilities.7−12 Inspired by nature, scientists have achieved a series of fruition in recent years, and several strategies have been proposed to induce preferential liquid directional transport, including asymmetric topography construction, heterowettability fabrication, and so on.3,4,13−16 Despite remarkable progress, the available products are far from what would be demanded for controlling a desired transport diversity due to limited transport amplitudes, weak controllability, and, especially, complex fabrication. Inherent challenges in unidirectional transport research stem from the simplification of the morphology and exploration of mechanism. Existing unidirectional liquid transport surfaces are based on microscaled, nanoscaled, and even molecular scaled © XXXX American Chemical Society
Received: May 24, 2018 Accepted: June 27, 2018
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DOI: 10.1021/acsnano.8b03924 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Fabrication and characterization of dual-scale A-shaped island arrays. (a, b) Schematic diagram of DLP 3D printing process (a) and layer-by-layer bottom-up additive manufacturing strategy (b). The inverted sample is pulled out of the liquid resin. The precured region (red rectangle) is between sample and screen. UV-LED light beam is projected onto bottom surface of sample through transparent screen, and liquid resin is UV-cured to form new layer on sample. Then, the sample is lifted along the +Z direction with a short distance, ΔZ, for each layer. In the next circle, the cured layer becomes substrate to form a new curing layer. Δτ is interval time of two continuous cycles. (c) Top view of the A-shaped island arrays model. (d, e) Stereomicroscope and magnitude SEM side view images of A-shaped islands. The island displays an italic letter “A” morphology (red dotted line) in top view. (f) Side view of the A-shaped island arrays model. L is the distance of between adjacent island, α is the titled angle between of islands and normal line, and H is vertical height of island. (g, h) Low- and highmagnitude side view SEM images of A-shaped islands. The edge of island has a stair-step shape and the height of a single step, h, is about 100 μm. Parallel microgrooves exist on the surface of the island from root to roof.
spontaneously in a single preferred direction and pin in all others without external energy input. Through digital light processing (DLP) three-dimensional (3D) printing technology,2,6,18 dual-scale arrays with parallel microgrooves can be fabricated on demand within 30 min. Through experiments and modellings, the relationship between liquid unidirectional transport and morphology parameters or liquid surface tension is explored systematically, and threshold values are determined. Significantly, desired microfluidics patterning, liquid shunting in target directions, antigravity unidirectional transport, and free combination of microfluidics device modules can all be realized on our well-design dual-scale arrays.
cured on the platform, the platform is subsequently lifted with a designed height, ΔZ, and gets ready for the next cycle. Repeating this layer-by-layer printing process with the microscaled shift between adjacent layers in different orientations, macroscale objects with microscale features are constructed. Figure 1c−h displays the morphology of the printed dualscale sample, that is, A-shaped macro-scaled island arrays with parallel microgrooves decorated on the island surface. The top view image of A-shaped island arrays model is shown in Figure 1c. Different from utilizing complex and expensive 3D printers, such as two-photon polymerization (2PP) and digital micromirror device (DMD) 3D printing methods, to get delicate morphologies, we choose an alternative layer-by-layer process through controlling Z axis resolution to construct macro-scaled island arrays with microscale microgrooves. The layer resolution, ΔZ, can be adjusted from 25 to 100 μm by adjusting the Z axis. As the sample can be controlled to decorate with repeat vertical side walls with a height of corresponding resolutions, the total fabrication time for our designed sample can decrease from more than 1 h for a resolution of 25 μm to less than 0.5 h for a resolution of 100 μm (Table S1). Compared with reported works,2,18 our strategy can save both fabrication time and cost, which brings the hope to construct macro-structured biomimetic materials through 3D printing process in an effective way. As Figure 1d reveals, the width of the individual island decreases gradually, which is shown as an italic letter “A”
RESULTS AND DISSCUSION In our experiments, we construct dual-scale A-shaped island arrays using DLP 3D printing methods. DLP 3D printer adopts a bottom-up additive manufacturing strategy. Figure 1a shows the device diagram of fabrication process, and Figure 1b shows the detailed schematic diagram. During the printing process, the 3D digital model of target sample is first sliced into multilayer 2D images. These images are then converted into 2D pattern and shown on the liquid crystal display (LCD) 4K screen. UV-light-emitting diode (UV-LED) light beams are projected through a screen onto the UV-polymerization interface at the bottom of the resin tank. As light triggers the polymerization process, the resin could be cured in an interval time Δτ at the polymerization interface between the holding platform and the tank window. After the first resin layer is B
DOI: 10.1021/acsnano.8b03924 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano
Figure 2. Liquid unidirectional transport on A-shaped island arrays. (a) Dyed oil is continuously deposited onto the top of island through a capillary tube. Deposited oil unidirectionally transports along +X direction and bypasses the next island one by one. (b) Detailed time-lapsed images of the unidirectional transport process between adjacent islands. This process can be divided into three steps. (c, d) Side view images and corresponding schematic illustration of processes decomposition of steps 2 and 3. Step 2: The capillary-driven directional transport process of precursor liquid (indicated by the red arrow) in the microgrooves. Step 3: Through liquid in microgrooves front and rear, attaching as connectors, internal Laplace pressure difference (P2 > P1) driven semilunar concave liquid level transports to +X direction unidirectionally, where d is the transport distance from the island root to the front of the liquid precursor, ω is the CA of the liquid on a flat photocurable resin surface, and β is the angle between P and the lever. Px is defined as the component of P along the X axes. Schematic diagram and equations of parameters relations are shown in the blue dotted line rectangle.
outline and sharp edge (red dotted line). A-shaped island arrays display special dual-scale micro/macro morphology: parallel microgrooves embed in macro-scale island surface (Figure 1e). In a side view of the model (Figure 1f), cuboidshaped substrate and periodic rhomboid islands constitute the main part of the sample. Three parameters are used to describe and restrict sample morphology: α is the titled angle between islands and normal line, L is the transverse spacing of islands, and H is the vertical height of islands. The morphology of typical sample, α = 60.0°, L = 2.0 mm, and H = 2.6 mm, can be detected directly by scanning electron microscope (SEM) images in Figure 1g,h. Like the sample model, islands grow slantwise on the substrate with constant thickness from root to top. The edge of the A-shaped island embodies a stepped appearance, and the height of each step is about 100 μm. Parallel microgrooves exist in the surface of island from root to roof, which coincides with the device setting and SEM top view image (Figure 1e). Comparing with the side view, the
cause of these microgrooves is the tiny shift and crack between adjacent layers, as interstices between the bricks of wall. From a certain perspective, the existence of microgrooves is the deficiency of layer-by-layer DLP 3D printing techniques. Through our optimization design, this deficiency is transformed into a beneficial key role, which will be elaborated on in the following text. Besides the surface with selected parameters as shown in Figure 1, A-shaped island arrays can also be fabricated with selected morphology parameters (Figure S1) by one-time printing without subsequent processing. A-shaped island arrays also exhibit great mechanical strength, which was preliminary tested as shown in Figure S2. Photocurable resin surface is superoleophilic with an oil, nhexadecane, contact angle (CA) of R2, so ΔP > 0 points to +X direction. As a result, oil transports continuously into island 2 through microgrooves.4,9,19 Similarly, as more liquid flows in, R2 gradually increases until it reaches the next island. Then, the liquid transport process enters the next circle. R also can be dsin β deduced by equation R = cos(β + ω) . Here, d is the transport
distance from the island root to the front of liquid precursor, ω is the CA of the liquid on flat photocurable resin surface, and β is the angle between P and the lever which is equal to π − 2α . Px 4 is defined as the component of P along the X axes (Px = P cos β). Finally, Laplace pressure differences along the X axes (ΔPx) are obtained:
( π4 − α2 + ω) π α Ld tan( 4 − 2 )
γ(L − d)cos ΔPx = P2, x − P1, x =
(2)
where ΔPx is the Laplace pressure difference along the X axes between adjacent meniscus liquid levels. ΔPx is the decline with the d gradual increasing, but it always points to +X directional. A micro/macro dual-scale structure for liquid rapid and continuous unidirectional transport is proposed. In the previous reports, almost all unidirectional liquid transport surfaces are based on microscale, nanoscale, and even molecular scale to obtain unbalance surface tension forces or complex topological structures. However, the unbalance forces arising from microscopic scale are too small to overcome the effect of contact-line pinning.15,19,22,24,25 So the transport is limited to a relatively short distance or low velocity. To overcome this limitation, our approach has achieved conceptually a unidirectional liquid transport on macroscale structure. D
DOI: 10.1021/acsnano.8b03924 ACS Nano XXXX, XXX, XXX−XXX
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Figure 3. Mechanism of switchable liquid transport. (a, b) Time-lapsed images of an oil transport process on A-shaped island arrays with gradationally changing α ranging (a) from 30.0° to 75.0° and (b) from 75.0° to 30.0°. (c) Time tracking statistics of the liquid transporting displacement on A-shaped island arrays with gradationally changing α. The open blue triangle represents the sample with incremental α. Deposited oil unidirectionally transports along the +X direction with increasing unidirectional displacement changing rate. The open green rectangle represents the sample with diminishing α, which displays switchable transport from +X to −X direction. Switching time, Δt, is α6, the rate of R6 change is relatively fast. Until R6 = R5, the transport distance (d3) is smaller than the adjacent island distance (L). At this moment, ΔPx = 0. Further increasing the content of oil, R6 begins to exceed R5, and the direction of ΔPx has been switched to the −X direction. This is also the fundamental reason for liquid transport direction switching. The theoretical analysis proves that the experimental result is coincident with the theoretical result. The Laplace pressure difference is the essential reason that causes liquid unidirectional transport in this A-shaped island array. Hence our work is able to achieve liquid transport with switchable velocity and direction only by gradationally changing α. Furthermore, compared to elegant common approaches that obtain asymmetric surfaces for liquid unidirectional transport such as chemical, temperature, electric force, magnetic field and others, our strategy has better application potential owing to the alleviation of external energy supply, controllability, and stability.13,22,26−31 Surface morphology parameters are one of the key factors to accomplish liquid unidirectional transport. In order to reveal the relationship between the two, a simple model is developed,
To prove the correctness of a Laplace pressure dominated unidirectional transport mechanism, A-shaped island arrays with incremental gradient α ranging from 30.0° to 75.0° (sample 1) and surface with diminishing gradient α ranging from 75.0° to 30.0° (sample 2) are designed and printed (Figure 3a,b. A liquid transport experiment has been tested on sample 1. Liquid can achieve rapid and consecutive unidirectional transport perfectly (Figure 3a). In contrast, for sample 2, the result shows that liquid will transport along the +X direction stepwise at the beginning, but, surprisingly, the liquid transport direction could be switched with the increase of the deposited liquid volume (Figure 3b). According to statistical results of displacement and velocity (Figure 3c and Figure S8), in either case, liquid displays the stepwise mode of transport with an impulse-type velocity. It can be clearly detected that liquid transport velocity is much faster in sample 1. However, the maximum velocity of two surfaces are both about 4.0−6.0 mm s−1. This is because velocity pulse interval of sample 1 is much shorter. Schematic illustration of this result is shown in Figure 3d. A theoretical analysis of sample 1 models that R4 gradually increases and liquid transports to the +X directional by the deposition of oil. Because of α4 > α3, the rate of R4 change is relatively slow. ΔPx (P4,x − P3,x) always points in the +X direction until the transport distance (d2) is larger than the adjacent island distance (L) (Figure 3d1). So, this transport E
DOI: 10.1021/acsnano.8b03924 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano
Figure 4. Threshold values of unidirectional transport. (a) Phase diagram of the relationship between surface morphology parameters and unidirectional transport abilities. Angle α of the fabricated A-shaped islands varies from 0° to 75.0° with different θ values, respectively. The open orange rhombus, green circles, and red triangles stand for experimental results of back-one unidirectional transport, unidirectional transport, and no unidirectional transport, respectively. Notice that back-one unidirectional transport represents liquid transporting along −X direction for only one island and then along +X direction, which can be classified into general liquid unidirectional transport. Inset: Schematic diagram explaining the geometries for the proposed model. (b) Measurement results of H2O/CH3CH2OH solution surface tensions (ST) and corresponding CA. The colors of these symbols indicate the relationship between liquid ST and unidirectional transport abilities. Blue dotted line represents the condition of unidirectional transport where θeq = 64.7°, which is obtained by experiments. Inset: Time-lapsed images of blue-dyed water and red-dyed ethanol spreading processes on A-shaped island arrays.
spread in the +X and −X directions simultaneously. But the spreading distance in the +X direction is larger than in the −X direction because of the existence of a meniscus liquid level. When the island is vertical (α = 0°), liquid spreads symmetrically in −X and +X directions. So, unidirectional transport cannot be achieved. Great spreading ability of oil on the surface (superoleophilicity) is one important condition to guarantee liquid unidirectional transport.12,18,19 It is reasonable for us to assume that all liquids which show a low contact angle can achieve unidirectional transport. So, ethanol with a low surface tension of 21.8 mN/m has been used to verify this hypothesis. Undoubtedly, experiments have proved that ethanol has a unidirectional transport ability on A-shaped island arrays with 1000 nL s−1 deposited speed (inset Figure 4b, red box). And the contact angle of ethanol on flat sample is
