Colloid Polym Sci.2016 Sep;294(9):1413-1423
Variable Wettability Control of a Polymer Surface by Selective Ultrasonic Imprinting and Hydrophobic Coating
Hyun-Joong Lee and Keun Park
Department of Mechanical System Design Engineering, Seoul National University of Science and Technology, Seoul, 139-743, Republic of Korea
Surface wettability is the ability of a liquid to maintain contact with a solid surface, and there have been many studies aimed at controlling surface wettability for development of functional surfaces. In this study, an approach to control surface wettability is proposed by differentiating surface energy of polycarbonate in specified regions, using a combination of two surface treatments: (i) micropattern replication using selective ultrasonic imprinting, and (ii) hydrophobic coating using organic silane. In both treatments, profiled masks were used to reduce surface energy in target regions where semi-hydrophobic polycarbonate surfaces were changed to hydrophobic states. Various combinations of selective ultrasonic imprinting and hydrophobic coating were investigated in terms of changes in surface wettability, by measuring contact angle (CA) behaviors on the developed surfaces. By controlling such changes, the surface wettability of regions on a single polycarbonate surface could be differentiated, allowing semi-hydrophobic (CA: 87.8o), hydrophobic (CA: 114.8o) and superhydrophobic (CA: 155.6o) states to coexist. This high CA difference (> 60o) on a single surface was then applied to enhance water repellent characteristics so that the surface wettability could be controlled effectively. Mechanical durability of the developed hybrid surface was also discussed by investigating its CA behavior after surface abrasion.
Ultrasonic imprinting uses ultrasonic vibration energy to soften a thermoplastic polymer surface and to replicate micropatterns. An ultrasonic imprinting system uses an ultrasonic horn and micropatterned mold, and acts as the following steps: (i) a polymer film is installed in the mold, (ii) the horn vibrates due to ultrasonic excitation, (iii) the horn presses the softened polymer film and (iv) the horn recedes and the patterned film is removed from the mold. In this procedure, ultrasonic vibration causes repetitive deformation and frictional heat on the polymer surfaces. Thus, the surface region of the target film is sufficiently softened that the micropatterns engraved on the mold can be replicated. This ultrasonic imprinting can be implemented on selective micropattern replication using a profiled mask film, as illustrated in Fig. 1. The mask film was fabricated in an annular shape, and was attached to the tip of the horn. Ultrasonic wave is then transferred from the horn to the target film through the mask film. Thus, in the target film, only the regions contacted with the mask film are affected by the ultrasonic excitation. Micropatterns can then be selectively replicated in these regions where the target film is locally softened .
Fig. 1 Configuration of selective ultrasonic imprinting using a profiled mask film
To develop hydrophobic micropatterns, a micromold containing a number of microscale holes was fabricated, as shown in Fig. 2a. The hole had a circular cross section of which diameter, pitch, and depth were 30, 75, and 20 mm, respectively. For the micromold fabrication, a silicon master was fabricated using MEMS processes including photolithography, deep reactive ion etching (DRIE), and electroplating processes . Figure 2b shows the micropatterned PC film, placed on a background sheet with printed numbers; it can be seen that the numbers in the patterned regions are not clearly seen due to the developed micropatterns. Figure 3c shows a scanning electron microscope (SEM) photograph of the developed micropatterns.
Fig. 2 (a) SEM photograph of the micromold, (b) micropatterned PC film, placed on a background sheet, and (c) SEM photograph of the developed micropatterns
To reduce the surface energy of the PC films, Trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane was coated onto the film surfaces. The silane coating was selectively applied on the film surfaces using profiled masks in order to reduce surface energy locally. This selective hydrophobic coating was conducted using a combination of selective ultrasonic imprinting for further wettability change. Thus, the silane was coated only for the micropatterned region as illustrated in Fig. 3, so that the surface energy of the patterned region could be further reduced to achieve superhydrophobic characteristics. Static contact angle (CA) was measured for various region, showing different CA values at different locations: 87.8o at the pure region (semi-hydrophobic state), 114.8o at the patterned region (hydrophobic state), and 155.6o at the patterned-and-coated region (superhydrophobic state).
Fig. 3 Selective surface modification for wettability control and the resulting CA comparison
To investigate the water repellent characteristics of the selectively patterned surface, water drop tests were performed for both plain and selectively patterned samples, with an increase in droplet volume. Figure 4 compares changes in water droplet shapes on both surfaces. The boundary curves for the patterned regions were marked with dashed lines for reference. In each case, the water droplet volume was changed from 10 to 70 mL, with an increment of 20 mL. This result indicates that the proposed surface treatments combining the selective ultrasonic imprinting and hydrophobic coating could improve water repellence significantly by reducing the surface energy in a selected region.
Fig. 4 Comparison of water repellence for the pure, patterned, and patterned-and-coated surfaces with an increase in droplet volume
These surface treatments are expected to have a variety of applications related to their ability to differentiate the wettability of regions of a single surface. Therefore, applications might include water-harvesting devices and partially developed functional surfaces; self-cleaning, anti-fogging and waterproof products. Considering that these surface modifications were performed using a single micromold and simple equipment setup, the proposed surface treatments combining selective patterning and coating can be used to control surface wettability, and thereby, to manufacture versatile functional surfaces efficiently.
- Jung W, Park K 2014 Selective ultrasonic imprinting for micropattern replication on predefined area. Ultrasonics 54: 4705-4715
- Lee HJ, Park K 2016 Development of hybrid surfaces with tunable wettability by selective surface modifications. Materials 9: 136
Acknowledgement: This research was funded by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education, Republic of Korea (Grant number: NRF-2013R1A1A2A10004709).
Keun Park, Ph.D.
Department of Mechanical System Design Engineering
Seoul National University of Science & Technology
172 Gongneung2-dong, Nowon-Gu
Seoul, 139-743, Republic of Korea