Article pubs.acs.org/JPCC
Rapid Fabrication of T‑Shaped Micropillars on Polypropylene Surfaces with Robust Cassie−Baxter State for Quantitative Droplet Collection An-Fu Chen and Han-Xiong Huang* Lab for Micro Molding and Polymer Rheology, The Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou 510640, PR China S Supporting Information *
ABSTRACT: Applying microinjection compression molding technology, a fast and flexible method is first proposed for the successive and mass replication of polypropylene surfaces with T-shaped micropillars in the present work. The water droplet on the titled surfaces grows in size to roll off due to gravitation effect. Interestingly, the roll-off angles on such surfaces are as a quadratic function of specified water droplet volume, meaning quantitative droplet collection and lossless transfer can be performed on the replicated surfaces. Meanwhile, self-cleaning behavior is preserved on the surfaces. Moreover, the robust Cassie−Baxter state on the replicated surfaces against the external pressure is demonstrated with droplet compression and immersion experiments. Specifically, a droplet sitting on the replicated surface can recover its spherical shape after squeezed to a water film as thin as 0.37 mm, and the replica is remained fully spotless after it is submerged into dyed water. The proposed method for fast replication of microstructured surfaces can be an excellent candidate for the development of microfluidics and droplet manipulation.
1. INTRODUCTION Some plant leaves in nature (such as lotus leaves and rose petals) exhibit super water repellency, resulting from a combination of micro/nanostructures and surface free energy.1,2 Water droplets immediately roll off the former surfaces due to low water adhesion, whereas they stick on the latter surfaces due to high water adhesion. Micro/nanostructured surfaces exhibiting moderate adhesion lying between the lotus and petal effects can be used to quantitatively collect and transport droplets.3,4 The microstructures on the surfaces play an important role in determining the wetting state of a solid surface.5 There exist two extreme wetting states on microstructured surfaces, that is, the Cassie−Baxter state6 and the Wenzel state.7 An energy barrier exists between the two extreme states, which varies depending on the surface morphology and chemistry.8 In the Cassie−Baxter state, the droplets do not penetrate into the gap between the micropillars on the microstructured surface due to the trapped air pockets. In the Wenzel state, the droplets penetrate into the gap and wet the microstructures, resulting in high adhesion without preserving superhydrophobicity. Once © 2016 American Chemical Society
such surfaces are exposed to external conditions such as droplet impact and hydrostatic pressure,9,10 metastable Cassie−Baxter state may transform to the Wenzel state. Thus, stability of the Cassie−Baxter state on biomimetic surfaces is urgently needed to be improved for their practical applications. Re-entrant structures on the surfaces can improve the stability of wetting state.11 The re-entrant structures may appear in terms of random patterns (nanoparticles,12−14 nanopores,15,16 nanowires,17 etc.) and regular patterns (microhoodoos,18 T-shaped micropillars,19−21 inverse trapezoids,22,23 textile fibers,24,25 etc.). For the random patterns, it is difficult to control geometric features and structure uniformity on large surfaces, although they may be prepared in a cost-effective way through chemical synthesis,26 electrospinning,18,25 or even candle soot deposition.12 Fortunately, the aforementioned T-shaped micropillars form robust composite interface and display contact angles (CAs) of >150° and low hysteresis, resulting in stable Cassie− Received: October 14, 2015 Revised: December 22, 2015 Published: January 5, 2016 1556
DOI: 10.1021/acs.jpcc.5b10079 J. Phys. Chem. C 2016, 120, 1556−1561
Article
The Journal of Physical Chemistry C Baxter state on the surfaces;19−21 however, T-shaped micropillars are mainly fabricated via a complex process, that is, SiO2 deposition on flat Si wafers followed by a two-step etching process comprising reactive ion etching of SiO2 and subsequent isotropic etching of Si.20,21 The method is faced with problems like harsh conditions, high cost, environmental unfriendliness, and a narrow selection for materials. Moreover, there is no report on the applications of surfaces with T-shaped micropillars in quantitative droplet collection. Considering the complex process of available preparation methods for surfaces with T-shaped micropillars, more facile methods should be developed. To pursue mass production of superhydrophobic micro/nanostructured surfaces, we regarded template method as an ideal choice, which can deliberately manipulate the size, shape, and arrangement of microstructures. 9,27,28 As a template method, microinjection compression molding (μ-ICM) combining microinjection molding (μ-IM) and nanoimprint lithography is widely used in high-efficient, high-precise, low-cost, and large-scale production and quite qualified for rapid fabrication.29−31 In previous works of our laboratory, the μ-ICM technology has been exploited in mass preparation of intricately microstructured superhydrophobic surfaces with high aspect ratio.32−34 Using μ-ICM, sticky superhydrophobic surfaces with T-shaped micropillars are prepared in this work, which stabilize the Cassie−Baxter state and can be used in quantitative droplet collection.
Figure 1. SEM micrographs of (a) template and (b, c) microstructured surface.
micropillar arrays form on the microstructured PP surface (Figure 1b,c). As can be seen, the top surface of the T-shaped micropillar is nearly a square with a length of ∼40 μm and the height of the micropillar is ∼83 μm. As shown in Figure 2, the melt fills the microcavities, overflowing into the microgap to form the T-shaped micropillars. The used PP material has good strength and ductility, and the wire mesh as a template is flexible. So, the micropillars can be detached from the template with slight deformation and without residue during demolding. Ruggedness appears on the top surfaces of the T-shaped micropillars due to no tight touch between the melt in the microgap and the mold cavity during melt filling and the slight deformation of the micropillars during demolding. 3.2. Droplet Adhesion and Quantitative Collection on Microstructured Surfaces. The 4-μL droplet dripping on the replicated microstructured surface exhibits a CA of 152.5° and an RA of >90°, as shown in Figure 3a. The 4 μL droplet is still pinned on the vertical replicated surface upon the addition of 1 μL microdroplet (see Supplementary Video S1). That is, classical petal effect appears on the vertical replicated surface for the droplet smaller than 5 μL. Furthermore, more microdroplets are added to analyze water adhesion characteristics of the replicated surfaces. Specifically, 2 μL microdroplets are continuously added to the original 4 μL droplet on the replicated surfaces tilted at six different angles (80, 70, 60, 50, 40, and 30°). The pinned droplets grow in size until they overcome the surface adhesion and roll off the surface under the gravitation effect (see Supplementary Videos S2−S7). The accumulated droplet volumes for rolling off at the six angles are obtained, and the results are shown in Figure 3b. Through fitting the data between 6 and 22 μL with a polynomial, an equation can be obtained as follows
2. EXPERIMENTAL SECTION 2.1. Materials. Polypropylene (PP; grade CJS700, China Petrochemical Co.) was used as received. 2.2. Mold and Replica Preparation. The μ-ICM experiments were performed on an 80-ton injection-molding machine (KM80SP180CX, Krauss-Maffei, Germany). A microinjection compression mold equipped with a temperature control apparatus was used to mold rectangular substrates with nominal dimensions of 120 × 20 × 0.7 mm3. As a template, a flexible twill-woven wire mesh (#500-mesh) with a thickness of 70 μm was mounted on the mold surface. Experiments were conducted at melt temperature of 230 °C, injection rate of 154 cm3 s−1, compression force of 280 kN, and mold temperature of 120 °C. It should be highlighted here that the molding cycle of each replica did not exceed 15 s. 2.3. Characterization. The surface topographies of the template and replicas were examined by scanning electron microscopy (SEM; Nova NanoSEM 430, FEI Co., Netherlands) at an accelerating voltage of 10 kV. Prior to the SEM observation, the template and replicas were coated with gold by ion sputtering. The CAs and roll-off angles (RAs) on each replica were measured using an automatic contact-angle testing apparatus (OCA 40, Dataphysics, Germany). Both CA and RA measurements were conducted at five different locations on the replica to obtain the average values.
y = 120.9 − 7.8x + 0.17x 2 , for 6 ≤ x ≤ 22
(1)
where x is the droplet volume (μL) and y is the tilted angle of the surface (degree). Large-sized droplets can be still pinned on the replicated surfaces tilted at angles of
