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Foolproof Method for Fast and Reversible Switching of WaterDroplet Adhesion by Magnetic Gradients Guanglei Hou,† Moyuan Cao,‡ Cunming Yu,† Shuang Zheng,§ Dianyu Wang,§ Zhongpeng Zhu,§ Weining Miao,† Ye Tian,*,§ and Lei Jiang† †
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China ‡ School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, P. R. China § Beijing National Laboratory for Molecular Science (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Science, Beijing 100190, P. R. China S Supporting Information *
ABSTRACT: Reversible switching of water-droplet adhesion on solid surfaces is of great significance for smart devices, such as microfluidics. In this work, we designed a foolproof method for fast and reversible magnet-controlled switching of water-droplet adhesion surfaces by doping iron powders in soft poly(dimethylsiloxane). The water adhesion is adjusted by magnetic field-induced structure changes, avoiding complex chemical or physical surface design. The regulation process is so convenient that only tens of milliseconds are needed. The on-site responsive mechanism extends its use to unusual curved surfaces. Moreover, the excellent reversibility and stability make the film an ideal candidate for real-time applications.
KEYWORDS: adhesion switching, fast, reversible, magnetic gradients, foolproof
1. INTRODUCTION Control over the wetting phenomenon is of great significance in both academia and industry.1−6 There are two kinds of distinct superhydrophobic adhesion surfaces in the natural world. Superhydrophobic surfaces with a “low-adhesion” property, such as lotus leaves,7−9 and superhydrophobic surfaces with a “high-adhesion” property, for example, rose petals10 and geckos’ feet.11,12 Reversible switching of liquid adhesion on the same substrate in response to external stimuli is crucial for smart materials, for example, for “no-loss” droplet transferring13,14 and for droplet-based microfluidic devices.15 To achieve controllable liquid adhesion, surface chemistry modification is the primary concern.16−20 However, the responsive chemical substances lead to a high cost and long response time, as well as low durability. Therefore, adjusting the surface morphology is more favorable for adhesion control. Among the methods for wetting control by surface morphology adjustment, such as mechanical pressure,15,21−23 temperature,24,25 humidity,26 and air sensing,27 the fast, noncontacting, and nontoxic magnetic gradients attract more attention in adhesion regulations.28−31 Flexible magnetic micropillars bend to a certain angle under an external magnetic field, leading to a switch between high- and low-adhesion states.32−35 However, patterned microstructures need more complicated preparation processes, which are time-consuming and costly. © 2017 American Chemical Society
In this article, we realized reversible magnetic control of pure water adhesion from the roll-off to the pinned state on magnetic soft composite films (MSCFs) with instantaneous response. A micro−nano-composite dual-scale surface morphology can be obtained by mixing poly(dimethylsiloxane) (PDMS) with heterogenous iron powders, followed by crosslinking and dissolution in organic solvent. Ferromagnetic iron powders are vulnerable to magnetic fields, and the induced magnetic force is strong enough to pull the soft elastic films upward to form striking vertical microstructures. The coneshaped microstructures conserve enough air to resist water penetration, leading to a superhydrophobic state. Furthermore, the sharp tips of the cone structures are also beneficial to the low-adhesive surfaces due to the reduction in the contact area with the water droplet. Once the magnetic field is removed, the vertical cone structures withdraw and recover to preliminary flat surfaces with a much higher adhesion, in tens of milliseconds. Ultimately, reversible adhesion on a single surface can be achieved by on/off switching of the magnetic field instantaneously. The preparation is so convenient that low-surfaceenergy materials (e.g., fluoroalkylsilanes) and the complex Received: May 25, 2017 Accepted: June 9, 2017 Published: June 9, 2017 23238
DOI: 10.1021/acsami.7b07409 ACS Appl. Mater. Interfaces 2017, 9, 23238−23245
Research Article
ACS Applied Materials & Interfaces
Figure 1. SEM images of the MSCF on a coverslip after cross-linking treatment. (a) Before and (b) after soaking in chloroform for 30 min; the inset shows an enlarged view of the chosen part, as labeled in the yellow dashed frame. The flat surface of the film was roughened after partial polymer dissolution in chloroform, and micro−nano-composite structures were prepared. (c) Cross-sectional image of the film. The optimal thickness is ∼50 μm. (d) Histogram of the statistics of size distribution of the iron powders. The percentage of iron powders with a size below 1 μm is approximately equal to that of iron powders with a size larger than 1 μm.
magnetic field and rearrange their positions along the magnetic vector lines. The Young modulus of cured PDMS 527 (E ∼ 1.5 kPa) is 3 orders of magnitude lower than that of the commonly used PDMS 18438 so that the composite films are soft enough to be pulled by the magnetic particles and form spikes at the surfaces when the applied magnetic field lines are vertical to the substrates. Figure 2 shows elongation of the spikes along the Z axis under various magnetic fields. When a magnetic field was absent (H = 0 T), the average height of the spikes was ∼15 μm, which can be inferred from the prevailing blue tagged areas and also the roughness profile image in Figure 2a. The line roughness of the film (Ra) was 1.804 μm, and the static contact angle (sCA) was ∼137.3°. The sCA is larger than that of the film that was not soaked in chloroform (110−120°, the intrinsic value for PDMS). This can be attributed to the higher roughness after solvent treatment. When H = 0.1 T, the spikes reach ∼20 μm. This can be attributed to a minor increase in the sCA to ∼139.5° and Ra to 2.162 μm. A similar growth progress was found at 0.2 T; however, the increase in both the sCA and Ra of the spikes from 0 to 0.2 T is negligible. The roughness at 0.2 T is merely 1.2 times higher than that at 0 T, whereas the increase in sCA is merely ∼4°. For a magnetic field of 0.3 T, a relatively significant increase in microstructures was observed, resulting in 32% augmentation of the Ra compared to that at 0.2 T, and sCA exceeds ∼146.8°; the nearly superhydrophobic state is beneficial to the low adhesion of water droplets. Thus, 0.3 T could be a threshold to realize adhesion control. The prevailing green surface indicates remarkable spike elongations at 0.4 T. According to the roughness profile, the average height of the spikes reached ∼35 μm and sCA exceeds ∼148.6°. Finally, some spikes with a height of more than 50 μm emerged at 0.5 T. The Ra is 3.489 μm, that is, almost 2 times higher than that in the absence of a magnetic field. The magnetic force
pattern fabrication are avoided, leading to an economical and eco-friendly technique. Further purification intended at collecting pure water is unnecessary. The on-site responsive mechanism extends its use to curved surfaces. Further, the excellent reversibility and stability make the film an ideal candidate for real-time application, such as droplet-based microfluidics.
2. RESULTS AND DISCUSSION The MSCFs were prepared by the doctor-blade coating technique and subsequent surface treatment. According to Figure 1a, the crosslinked iron−PDMS composite film without treatment with an organic solvent possesses a defect-free surface and great homogeneity. The film shows a rough surface after chloroform treatment (Figure 1b). The profile of the iron particles covered by a thin polymer film was exposed after partial dissolution of outermost layers of PDMS. Further research reveals that the optimal thickness of the film is ∼50 μm (Figure 1c). Thicker films are unable to respond to external stimuli, whereas it is difficult for thinner films to form large enough vertical structures (Figure S3). The sizes of the iron particles are summarized in Figure 1d. It is worth noting that the amounts of iron particles with submicrometer diameters are approximately equal to those of iron particles with micrometer diameters. Composite structures on submicrometer and micrometer scales are shown in the inset of Figure 1b. Thus, the heterogeneous sizes of the iron powders endowed a micro− nano dual-scale roughness to the surfaces. Previous reports have proven that micro−nano-composite dimensions are beneficial in wetting control.36,37 Elastic film and force loading points are essential to the responsive morphological changes. As already known, ferromagnetic powders can be magnetized easily with an external 23239
DOI: 10.1021/acsami.7b07409 ACS Appl. Mater. Interfaces 2017, 9, 23238−23245
Research Article
ACS Applied Materials & Interfaces
Figure 2. sCAs and roughness profiles as well as real-time three-dimensional confocal microscopy images of the MSCFs under various magnetic fields. (a−f) Images captured at 0, 0.1, 0.2, 0.3, 0.4, and 0.5 T, respectively. The roughness of the surface increased with an increase in the magnetic field. From 0 to 0.2 T, negligible structural changes were observed. When the applied field exceeded 0.3 T, tremendous spikes emerged due to the magnetic volume force being strong enough to conquer the elastic modulus of the film. Both the sCA and Ra reached maximum values at 0.5 T, and the superhydrophobic state was obtained at this time. Ra denotes to the line roughness of the local profiles. The units of all coordinate systems are micrometers (μm).
roll off or bounce at a tiny tilting angle. Thus, reversible switching of water-droplet adhesion on one surface is achieved by alternate magnetic field modulation. This results reflect the monotonous change in adhesive forces with the applied magnetic fields. The scheme of reversible switching is described in Figure 3d. In the absence of a magnetic field, the water droplet could penetrate into the small bumps easily and wet the bottom surface without air-pocket obstruction. The wetting state at this time is considered as the “wenzel state”, and a large contacting area results in high adhesion. When a strong magnetic field was applied, micro−nano-composite scale spikes with a cone shape emerged along the vertical magnetic lines. According to Figure 2, the spikes are more than ∼30 μm in height. A water droplet sitting on the surface of the spikes will not penetrate into them due to the stable air pocket. Furthermore, the cone shape with sharp tips is beneficial in reducing the liquid−solid contacting areas, leading to a lowadhesive state. However, it is worth noting that without modification of low-energy materials, the intrinsic high adhesion of PDMS is somehow helpful in the adherence of the water droplets, and the water droplet could wet the upper part of the spikes. The wetting state here is the “transitional state” rather than the ultra-low-adhesive “cassie state”, like the lotus effect.
exerted on iron particles could drag the elastic PDMS layer, forming many spikes. More air could be reserved in the valley between the spikes, promoting the surface to reach a superhydrophobic state of sCA ∼ 151.4°. For intuitively showing the droplet’s behavior under different adhesion conditions, the MSCF was placed at a tilting angle of 9°. When H = 0 T, the water droplet fell down and pinned to the initial position immediately (Figure 3a). This denotes a high water-droplet adhesion of films when no magnetic field is applied. On the other hand, when the magnet was applied underneath the same film, the water droplet bounced up into the air 25 ms after the first impact. Then, the water droplet fell back onto the surface at ∼50 ms and bounced up again after 75 ms (Figure 3b). The bouncing action of droplet implies an excellent low-adhesion property of the film at H = 0.5 T. We also examined the exact adhesive force (Fa) of the film for water droplets using a microelectromechanical balance system, as shown in Figure 3c; when H = 0 T, Fa ∼ 120 μN. This is slightly lower than intrinsic PDMS adhesive force for water and is considered to be a comparatively high adhesion state. When a 0.3 T magnetic field was applied, the Fa decreased obviously to ∼70 μN. Finally, the Fa decreased to ∼30 μN at 0.5 T. In the absence of a field, Fa recovered to ∼120 μN immediately. Although the surface is not superslippery at H = 0.5 T, the adhesion of the surface is low enough to make a water droplet 23240
DOI: 10.1021/acsami.7b07409 ACS Appl. Mater. Interfaces 2017, 9, 23238−23245
Research Article
ACS Applied Materials & Interfaces
Figure 3. Time sequential images showing the pinning or bouncing of a water droplet impacting on an MSCF under (a) 0 T and (b) 0.5 T magnetic fields. When a magnetic field is absent, the water droplet will get pinned at the local position after impacting the surface. Meanwhile, the water droplet will bounce upward after impacting the surface in the presence of a magnetic field beneath due to low adhesion. (c) Water-droplet adhesion force (Fa) of the films under different magnetic fields. Fa decreased with increasing magnetic field and reached ∼30 μN at 0.5 T compared to ∼120 μN at 0 T. (d) Scheme of the reversible adhesion switch between two wetting states. The tilting angle of the films is 9°; the volume of the water droplets is 4 μL.
surface very easily, indicating a slippery surface acquired by dynamic magnetic switching. The stability and duration of the system have also been examined here. As shown in Figure 5a, the reversibility of the MSCF was examined by testing the adhesive forces for water droplets through the ON/OFF magnetic field alternatively. Obvious and stable distinctions in the adhesive force last for 10 cycles. The adhesive forces at 0 T/0.5 T are ∼120 and ∼30 μN, respectively. The stability test is performed ex situ here due to the difficulty in wetting switch from the “wenzle state” to “cassie state”. We believe that not only 10 cycles but hundreds of cycles would be possible with our device. The duration is also shown in Figure 5b. We define Δ as the ratio of adhesive forces at the 0 T/0.5 T magnetic field, reflecting the control ability of adhesion switches. The ratios are equal to ∼4 constantly (∼120/∼30 μN), even 180 days past without any decay since the film was prepared. The mechanical properties are also examined in Figure 5c; the stress−strain curve (red line) indicates that the film has a yield strength of ∼14.5 kPa and an ultimate tensile strength of ∼79.5 kPa. The elastic elongation is ∼41.5%, and the elongation at break is ∼129.7%. This demonstrates that the film is very soft, with a great elastic property. Thus, the film is sensitive to the magnetic field and can be stretched to form obvious spike microstructures. The peeling strength is ∼134.3 kPa (blue line), indicating that the system has excellent film−substrate adhesion properties and will not be easily destroyed upon regular use. The Si−O−Si
We examined the adhesive force of the MSCF on the basis of whether a water droplet could roll off from a tilted surface under various magnetic fields. When the gravitational component overcomes the adhesive force, the droplet starts to move. The rolling behavior is also related to the tilting angles and magnetic field strength. At a small tilting angle, the rolling behavior is related to differences in the droplet volume. As shown in Figure 4, regardless of the droplet volume, total pinning is observed when the magnetic field is ≤0.2 T. It is clear that the spikes that are not tall enough under a weak magnetic field are attributed to the high adhesive force for water droplets. When a 0.3 T field was applied, small water droplets (
