Wetting characteristics of bare micro-patterned cyclic olefin copolymer surfaces fabricated by ultra-precision raster milling

Abstract

The sliding performance of hydrophobic micro-patterned surfaces is one of the major factors determining wettability. However, it is difficult for existing manufacturing methods such as lithography, laser etching, and chemical reaction to fabricate one-step bare hydrophobic micro-patterned surfaces with good sliding performance for mass production. In the present research, one-step fabrication of bare hydrophobic micro-grooved and micro-pillar cyclic olefin copolymer surfaces with high precision in geometries has been achieved by ultra-precision raster milling (UPRM). According to the comparison of the static contact angle with theoretical models, droplet anisotropy, droplet contact line, contact angle hysteresis, and sliding angle measurement from the experiment, it is found that the droplet under the Cassie and Baxter regime gives a good sliding performance on bare micro-directional grooved cyclic olefin copolymer surfaces due to the shape edges induced by the numerically-controlled tool path of the material removal process in mechanical machining. It is believed that the micro-directional grooved surface has great potential for mass production by plastic injection molding in microfluidic applications such as artificial self-cleaning surfaces.

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1. Introduction

Microstructures in nature, such as lotus leaves and butterfly wings, have micro-patterned surfaces that possess hydrophobic properties.^1–3 These hydrophobic surfaces have great potential application in biomimetic self-cleaning surfaces due to their extreme water repellence.^4–6 Recent studies have focused on the fabrication of hydrophobic micro-patterned surfaces using various methods such as lithography,^7,8 laser etching,^9,10 chemical reaction,^11,12 electrospinning,^13,14 and self-assembly.¹⁵ Most of the manufacturing processes are complicated due to chemical treatments, time-consuming, not suitable for fabricating large surfaces, or only applicable to several engineering materials such as metals, semiconductors, and alloys but limited to plastics. Sliding performance of hydrophobic micro-patterned surfaces is one of the major factors determining wettability.^16,17 However, there are very little one-step manufacturing methods capable to fabricate bare hydrophobic micro-patterned surfaces with good sliding performance for mass production.

Mechanical machining would be one of the best solutions to fabricate bare hydrophobic micro-patterned surfaces with good sliding performance for mass production due to its numerical control of machine tool path during the material removal process. Ultra-precision machining such as fast-tool-servo diamond turning, micro-grinding, ultra-precision raster milling (UPRM), are capable of manufacturing micro-patterned surfaces with sub-micrometer form accuracy and nanometer surface roughness without subsequent processes. Although fast-tool-servo diamond turning is capable to generate different micro-patterned arrays, the aspect ratio of the generated micro-patterned arrays is relatively low.¹⁸ It is difficult for droplets on these micro-patterned arrays to form a composite solid–liquid–air interface which is essential for hydrophobic surfaces to achieve self-cleaning properties.⁶ Li et al. recently reported a mechanical micro-grinding method to fabricate hydrophobic micro-V-grooved Si surfaces.¹⁹ However, the geometrical dimensions of the micro-V-grooves are not highly controllable and this method can only be applied to the machining of some engineering materials such as metal, alloy, or ceramic, but not to plastics. Ultra-precision raster milling has been conventionally applied to machine mold inserts for F-theta lenses, V-groove structures for fiber array connector, and other optical freeform surfaces. It takes advantages to manufacture both the bare hydrophobic micro-patterned surfaces and its mold insert for mass production in plastic injection molding. This greatly reduces the cost and time for manufacturing hydrophobic micro-patterned surfaces. It is known that a sample surface with a high static contact angle with droplets does not mean that it has good droplet mobility.²⁰ Although Kong et al. has proposed a framework for the design, fabrication, and characterization of three dimensional patterned microstructured surfaces with a high static contact angle of water on hydrophilic materials, both the fabrication details and the wettability of the machined micro-patterned surfaces are still incomplete.^21,22 Thus, there is still a lack of a highly controllable fabrication method of bare micro-patterned surfaces with good sliding performance for mass production.

This paper studied the wetting characteristics of bare hydrophobic micro-patterned surfaces machined by a highly controllable one-step mechanical machining method, UPRM which is potentially capable to manufacture bare hydrophobic micro-patterned surfaces with good sliding performance for mass production in plastic injection molding. In the experiment, a micro-directional groove surface and a micro-pillar surface are fabricated by UPRM. According to the mathematical models of micro-patterned surfaces, it is found from the experiment that the measured static contact angle of a water droplet agree with the static contact angle predicted by the models. This shows that the experimental data successfully validate the models. More importantly, the micro-patterned surfaces with high precision in geometries generated from UPRM help define a more precise area fraction of the projected wet area for predicting the static contact angle accurately, and for investigating the feasibility of the application of the Wenzel as well as the Cassie and Baxter model. By studying static contact angle, interaction between the droplet and the sample surface, contact angle hysteresis, and sliding angle of water droplet on micro-patterned surfaces machined by UPRM, the droplet governed by the Cassie and Baxter regime are proved to have good sliding performance. These bare micro-patterned surfaces are shown to have great potential for mass production in microfluidic applications such as artificial self-cleaning surfaces.

2. Experimental details

2.1 Experimental setup

To prepare flat sample surfaces for machining micro-patterns, single-point diamond turning (SPDT) was employed to cut sample surfaces at the same level by a Nanoform 200 machine at the beginning. This avoids the occurrence of various depths of micro-patterns on a large area of sample surface due to the incorrect level of different samples. After the preparation, a Precitech Freeform 705 G (Precitech Inc., USA) 5-axes CNC ultra-precision raster milling machine as shown in Fig. 1(a) was applied to generate micro-patterned surfaces on the sample surfaces which were attached by a specially designed fixture. During the machining process, a single crystal diamond facet cutting tool with specific included angle and width of tool tip was selected and fixed on the spindle with a certain swing distance. Fig. 1(b) shows photos of the single crystal diamond facet cutting tool with a 30° included angle, a 75° exterior angle (β), and a 14.0 μm tool tip width taken by a scanning electron microscope (SEM). The cutting conditions are as follows: spindle speed = 4000 rpm; depth of cut = 4 μm; swing distance = 30.665 mm; feed rate = 100 mm min⁻¹; and step distance = 0.05 mm in vertical cutting. The sample material for this study is cyclic olefin copolymer (Topas COC, 5013SL-01) which is copolymerized from norbornene and ethylene with a metallocene catalyst. All the 20 mm × 20 mm × 5 mm COC sheets used in the experiment were prepared by plastic injection molding.

Fig. 1 (a) Cutting geometry of UPRM, and (b) SEM images of a diamond facet cutting tool tip.

2.2 Fabrication of micro-patterned surfaces

In UPRM, there are two cutting strategies which are horizontal cutting and vertical cutting. The main difference of cutting geometry between these two cuttings is that the feed direction and the raster direction in horizontal cutting are opposite to that in vertical cutting as shown in Fig. 2. After the consideration of cutting efficiency and surface quality, a micro-directional grooved surface with 10 μm depth, 45 μm width, 45 μm spacing, and a 75° angle were fabricated by the tool path planning for surface generation. Unidirectional retreat method was employed to generate grooves on the whole single point diamond turned sample surface by horizontal cutting strategy as shown in Fig. 2(a). After the generation of the whole micro-grooved surface on the sample, vertical cutting with a step distance (i.e. 0.05 mm) were then employed to mill the whole sample surface by moving up the cutting tool with a specific value on the z-axis. The vertical cutting not only controls the depth of grooves but also can control the roughness at the top of the ridges using different cutting conditions such as feed rate and step distance.²³

Fig. 2 Tool path design of surface generation in (a) micro-directional grooved surfaces, (b) micro-pillar surfaces, and (c) cutting cycles of grooves in horizontal cutting strategy.

For the generation of micro-pillar surfaces, horizontal cutting strategy using a unidirectional retreat method was employed to first generate grooves in one direction as illustrated in Fig. 2(a). After the generation of the whole micro-grooved surface on the sample, the fixture was then rotated 90° along the z-axis. Horizontal cutting strategy using a unidirectional retreat method was employed to generate grooves which are perpendicular to the previous direction.

The combination of the generated micro-grooves perpendicular to each direction forms a micro-pillar surface. Vertical cutting with a step distance of 0.05 mm was then employed to mill the whole sample surface by moving up the cutting tool a specific value along the z-axis as shown in Fig. 2(b). Due to the dimension differences between the geometries of the grooves and the cutting tool, and the protection of the diamond cutting tool from wear, different numbers of cycles of cutting were used to generate an individual groove. Fig. 2(c) shows a schematic diagram of each groove that was machined in four cycles by a facet diamond tool. The depth of rough-cut in cycle 1 and 2 is over 10 μm, whereas the depth of finish-cut in cycle 3 and 4 is 4 μm. It is known that cutting conditions and machine tool characteristics are major factors affecting the surface generation in UPRM. Therefore, surface quality evaluation is necessary and it will be discussed in the next session.

2.3 Micro-patterned surface topography measurement

To measure the geometrical parameters of micro-directional grooved and pillar surfaces machined from the experiment, a white light interferometer (Zygo Nexview) was employed to capture their surface topographies. Fig. 3 shows the surface topographies of the micro-directional grooved and pillar surfaces in 20× magnification respectively from the experiment. It is found that the deviations between the designed and the measured dimension of micro-grooved and pillar surfaces are within ±2 μm as listed in Table 1. The measured angles of the slopes are around 75°, which is nearly equal to the angle formed by the tool geometry. The deviations between the designed and measured height of pillars may be due to the material recovery of cyclic olefin copolymer. At the bottom of grooves as shown in Fig. 3(b), there is a step which shows a difference value of pillar height, around 0.6 μm, in cross section B–B and C–C. The measured experimental results show that the rotation of the designed fixture did not significantly affect the relative depth of cut by the tool into the sample surface based on the datum of fixture when applying horizontal cutting for generating the micro-pillars.

Fig. 3 Surface topographies of (a) the micro-directional grooved surface, and (b) the micro-pillar surface from the experiment.

Table 1 Measured geometrical parameters of micro-patterned COC surfaces

Cross section	Design depth (μm)	Design width (μm)	Design spacing (μm)	Measured depth (μm)	Measured width (μm)	Measured spacing (μm)	Measured angle, β (°)
A–A	10	45	45	10.0	45.5	45.1	75.3
B–B	10	45	45	8.6	44.7	45.9	74.7
C–C	10	45	45	8.0	45.5	45.1	75.2

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3. Wetting characteristics of micro-patterned surfaces

3.1 Comparison with theoretical models

3.1.1 The Wenzel model. The Wenzel model describes a droplet that completely wetted a rough surface and formed a homogeneous solid–liquid interface as shown in Fig. 4. The static contact angle of the droplet under the Wenzel regime, θ_W can be calculated as follows:²⁴

cos θ_W = r cos θ_Y (1)

where r is the roughness ratio on the rough surface, and θ_Y is the static contact angle of a droplet on a flat surface. As shown in Fig. 4, the shaded regions and the dotted line regions represent the wetted area A_w and the projected area of the wetted regions A_proj on the micro-patterned surface respectively.

Fig. 4 Schematic illustrations of the wetted area A_w and the projected area of the wetted regions A_proj on (a) micro-grooved surfaces, and (b) micro-pillar surfaces.

The roughness ratio, r of the micro-grooved surface is:

where s is the spacing between grooves, w is the width of grooves, h is the depth of grooves, β is the exterior angle induced by the facet cutting tool, and l is the length of grooves on micro-grooved surfaces as illustrated in Fig. 4(a).

The roughness ratio, r of the micro-pillar surface can be derived as follows:

where A_i is the total wetted area of A₁, A₂, A₃, A₄ and A₅, s₁ and w₁ is the spacing and width of pillars in one direction respectively, and s₂ and w₂ is the spacing and width of pillars in the direction perpendicular to the previous direction respectively as illustrated in Fig. 4(b).

3.1.2 The Cassie and Baxter model. The Cassie and Baxter model describes a droplet on a heterogeneous surface with solid–liquid–air interface as shown in Fig. 4. The static contact angle of the droplet under the Cassie and Baxter regime θ_CB can be calculated as follows:²⁵

cos θ_CB = R_ff cos θ_F + f − 1 (4)

where f is the area fraction of the projected wet area, and R_f is the roughness ratio on the wet area. As shown in Fig. 4, the shaded regions and the dotted line regions represent the wetted area A_w and the projected area of the wetted regions A_proj on the micro-patterned surface respectively. The roughness ratio R_f on the wetting area of top asperities on both the micro-grooved and pillar surfaces are assumed to be 1 in eqn (4) since a facet diamond cutting is applied to generate an optical flat surface by vertical cutting.

The area fraction of the projected wet area, f of the micro-grooved surface is derived as:

The area fraction of the projected wet area, f of the micro-pillar surface is derived as:

By using eqn (2) and (5), the static contact angles from the Wenzel model as well as the Cassie and Baxter model on the micro-grooved surface are predicted. Similarly, the static contact angles from the Wenzel model as well as the Cassie and Baxter model on the micro-pillar surface are predicted by using eqn (3) and (6).

3.2 Static contact angle from the experiment

In order to validate the mathematical models of the micro-directional grooved and pillar surfaces built in the previous section, static contact angles of water droplets have been measured by the sessile drop method. The images of water droplets on various sample surfaces captured by camera from side view are presented in Fig. 5. Each reported data of the static contact angle is the average of six to eight independent measurements by a deionized water droplet of approximately 5 μl. Table 2 summarizes the experimental results and the calculation from the Wenzel model as well as the Cassie and Baxter model using wetting analysis of micro-patterned surfaces.

Fig. 5 Static contact angles of water droplets on (a) the flat surface, (b) the micro-grooved surface, (c) the micro-pillar surface, and (d) air pockets under the droplet on the micro-patterned surface.

Table 2 Wetting performance of micro-patterned COC surfaces

Surface pattern	Observation direction	Calculated θW	Calculated θCB	Average contact angle, θ	Sliding angle (°)
Groove	Parallel	88.0°	119.0°	θ‖ = 108.9° ± 1.6	24°
	Orthogonal	88.0°	119.0°	θ⊥ = 145.9° ± 1.8	Not slide
Pillar	Parallel	87.7°	138.0°	θ‖ = 137.8° ± 1.6	Not slide
	Orthogonal	87.7°	138.0°	θ⊥ = 138.1° ± 2.0	Not slide
Flat	—	—	—	θ = 88.3° ± 1.9	Not slide

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The static contact angle of water droplet on a single point diamond turned flat COC surface is 88.3° due to its intrinsic hydrophobic property, as shown in Fig. 5(a). After the generation of micro-directional grooves on the flat COC surface by UPRM, the static contact angle of water droplet increases from 88.3° to 108.9° in the parallel direction, as shown in Fig. 5(b). This reveals that the droplet freely advances on the ridges in the parallel direction, but is restricted by the periodic ridges that induce an energy barrier in the orthogonal direction.^26,27 The average static contact angle of water droplet therefore greatly increases from 88.3° to 145.9° in the orthogonal direction. With respect to the micro-pillar COC surface, the static contact angle of water droplet increases from 88.3° to 137.8° and 138.1° in two directions perpendicular to each other as shown in Fig. 5(c). This indicates that the measured static contact angle of water droplet is greatly in agreement with the static contact angle of 138.0° predicted by the Cassie and Baxter model. More importantly, this shows that UPRM is capable of manufacturing micro-patterned surfaces with high precision in geometries which define a more accurate area fraction of the projected wet area to predict the static contact angle by the Cassie and Baxter model compared with other conventional fabrication methods such as lithography, laser etching, and chemical reaction.^7,10,11 It is difficult for these conventional methods to manufacture micro-patterned surfaces with highly controllable precision in geometries and this significantly leads to the area fraction of the projected wet area in the Cassie and Baxter model not being clearly defined. Fig. 5(d) shows that air pockets exist under the droplet on micro-patterned surfaces. This proves that the droplet is under a composite wetting state.

3.3 Droplet anisotropy

To observe the anisotropy of the water droplet on various COC surfaces including a flat surface, a grooved surface, and a pillar surface, water droplet contour was captured by an optical microscope from the top view as shown in Fig. 6(a)–(c). The droplet shape on the flat COC surface is circular shape which is nearly the same as its initial shape from the deposition by a syringe. On the grooved surface, the droplet shape is elliptical which reveals that the droplet becomes slightly anisotropic due to the directional grooved surface geometries on the sample. Compared with the grooved surface, the droplet shape on the pillar surface is more rectangular. This reveals that the droplet becomes more isotropic due to the square pillars with equal spacing of the surface geometries on the sample.

Fig. 6 Anisotropy of a droplet on (a) the flat surface, (b) the micro-grooved surface, and (c) the micro-pillar surface from the top view.

3.4 Droplet contact line

In order to quantify the droplet anisotropy, the lengths of the contact lines of the droplets on different samples in the parallel and orthogonal directions from the side view in Fig. 5 have been measured using software named ImageJ. The length of the contact line is 3.249 mm on the flat surface. On the micro-directional grooved surface, the lengths of the contact lines in the parallel and orthogonal directions are 2.083 mm and 1.274 mm respectively. On the micro-pillar surface, the lengths of the contact lines in the two directions are both equal to 1.471 mm. These experimental results indicate that the micro-patterns have greatly reduced the lengths of the contact lines in the parallel and orthogonal directions compared with the flat surface.

In addition, the droplet on the micro-directional grooved surface elongated more in the parallel direction than that in the orthogonal direction. This can be explained by that the droplet under the Cassie and Baxter state on the micro-directional grooved surface freely advanced at the top of the ridges in the parallel direction after its deposition, but it was restricted by the spacing between the pillars in the orthogonal direction. On the micro-pillar surface, the droplet under the Cassie and Baxter state was restricted by the equal spacing between the pillars in the parallel and orthogonal directions. The air in between the pillars induced equal amount of energy barrier to the droplet, and this results in the same length of the contact line. As a result, the droplet on the micro-pillar surface is more isotropic than that on the micro-directional grooved surface.

According to the image of the water droplet contact line from the top view as shown in Fig. 7, the water droplet is sitting at the top of the ridges and the pillars on micro-directional grooved and micro-pillar surfaces respectively. This may be due to the shaped edges induced by the machining process of vertical cutting on the ridges and pillars, which help stabilize the deposition of water droplet. In other words, the shaped edges make the droplet sit at the top of the micro-pillars due to the pinning effect. This provides the evidence that the droplet is governed by the Cassie and Baxter regime, and forms solid–liquid–air interface with the micro-patterned surface. More importantly, this is one of the essential requirements to achieve hydrophobic self-cleaning surfaces.

Fig. 7 Water droplets acting on (a) the micro-grooved surface, and (b) the micro-pillar surface.

3.5 Contact angle hysteresis

In order to quantify the directional adhesion of the droplet anisotropy on three different samples, the advancing and receding angles of the water droplets were measured to calculate contact angle hysteresis (CAH).²⁸ The droplets were being dragged by a syringe on the flat surface, the micro-grooved surface, and the micro-pillar surface as shown in Fig. 8. In the case of a droplet not moving at the advancing and/or receding contact line(s), the angle(s) measured at the front and/or at the back is/are named as “distorted angle”.

Fig. 8 Snapshots of the advancing and the receding angle measurement on (a) the flat surface, (b) the micro-grooved surface, and (c) the micro-pillar surface.

According to the experimental results, only the droplet on the micro-grooved surface in the parallel direction moved at the advancing and receding contact lines when being dragged as shown in Fig. 8(b). All the other droplets were either distorted on its original position, or just advanced but not receded on the sample surface. These reveal that the droplet mobility of the micro-grooved surface in the parallel direction is higher than that on both the flat surface and the micro-pillar surface. This can be explained by that the droplet under the Cassie and Baxter regime on the micro-grooved surface in the parallel direction can freely advance and recede at the top asperities of the continuous ridges when the droplet is being dragged.^26,28 These free movements of the advancing and receding contact lines on the continuous ridges result in the relatively low advancing and receding angles (106° and 71°) in the measurement as shown in Fig. 8(b) when compared to that in the orthogonal direction and that on the micro-pillar surface. The CAH of the micro-grooved surface in the parallel direction is 35°. However, the droplet on the micro-grooved surface in the orthogonal direction was highly restricted by the spacing between the continuous ridges. Some researchers indicated that surface friction is the main obstacle to the droplet mobility on rough surfaces.²⁶ It is believed that surface friction also plays a key role in this study. Thus, the friction induced by the continuous ridges acting on the droplet results in the highest advancing angle (162°), and distorted angle at the back (130°) as shown in Fig. 8(b).

In respect to the micro-pillar surface, the droplet under the composite wetting state just advanced but not receded on the sample surface in both the parallel and orthogonal directions. The reason is that the movements of the droplet contact line in the two different directions were restricted by the equal spacing between the pillars.²⁹ This makes that the droplets require a similar amount of activation energy to overcome the barrier of the wetting and dewetting movements induced by the pillars at the advancing and receding contact lines when being dragged. All of these results in similar advancing angles (153° and 158°), and distorted angles at the back (98° and 105°) in the parallel and orthogonal directions respectively as shown in Fig. 8(c). Therefore, the differences between the advancing angle and the distorted angle at the back in the two directions are similar (55° and 53°).

Regarding the flat surface, the droplet under the complete wetting state was only distorted but not moved at the advancing and receding contact lines when being dragged as shown in Fig. 8(a). This implies that the adhesion force between the droplet and the surface is much higher than that between the syringe and the droplet. Thus, this leads to the droplet with 88.3° average static contact angle remaining on its original position, and results in the distorted angle at the front (92°), and the distorted angle at the back (77°). It is believed that the droplet wetting state is the dominant factor in this case.

3.6 Sliding angle measurement

To investigate the droplet mobility on sample surfaces, sliding angle measurements were conducted on the flat surface, the grooved surface, and the pillar surface. Sliding behaviour of a water droplet on a sample surface is among the fundamental result of wettability.^16,17 Sliding angle is the characteristic of the contact angle hysteresis of the sample surface. Sliding angle is defined as the angle at which the droplet starts to move on the sample. During the sliding angle measurement, a 10 μl water droplet was firstly deposited on the sample surface on a computer-controlled tilting stage in a horizontal position. The tilting angle then keeps increasing until the droplet rolls off the surface. Fig. 9 shows the snapshots of videos that recorded the sliding angle measurement of various sample surfaces. With respect to the flat COC surface, the water droplet did not slide even at a 90° tilting angle as shown in Fig. 9(a). However, the water droplet on the grooved surface started to slide at 24° in the parallel direction, as shown in Fig. 9(b). Compared with the grooved surface, the water droplet on the pillar surface did not slide even at a 90° tilting angle, as shown in Fig. 9(c). This reveals that the water droplet under the Cassie and Baxter regime still forms a solid–liquid–air interface with the micro-directional grooved surface due to the shape edge of the ridges during the tilting process as shown in Fig. 9(b).

Fig. 9 Snapshots of water droplets acting on the inclined (a) flat surface, (b) micro-grooved surface in the parallel direction, and (c) micro-pillar surface.

The sliding mechanism of a water droplet on micro-pillar surfaces is completely different from that on micro-directional grooved surfaces. The water droplet on the micro-pillar surface experiences a discrete wetting and dewetting process at the advancing and receding contact line respectively due to the spacing between the pillars, whereas the water droplet on the micro-directional surface experiences a continuous wetting and dewetting process which is related to the mass of inertia of the droplet when sliding on the continuous ridges in the parallel direction. Thus, the discontinuous advancement of the droplet caused by the spacing between the pillars induced energy barrier which results in the droplet not sliding on the inclined micro-pillar surface even at a 90° tilting angle, whereas the water droplet on the micro-grooved surface slides at 24° in the parallel direction due to the free advancement of the droplet on the continuous ridges.

Tilting a sample to an inclined angle is a dynamic situation which is different from the static contact angle measurement by the sessile drop method. Theoretically, although the contact area between the water droplet and the pillar surface is less than that on the grooved surface, one of the edges of the micro-pillars may act as an opposite force on an inclined surface to the sliding direction of the water droplet. This may greatly increase the friction acting on the water droplet. Additionally, the geometries of the micro-pillar surface provide more spacing for the water droplet penetrating in between the pillars. This may trigger the collapse of droplet and its wetting state transited from the Cassie and Baxter regime to the Wenzel regime during tilting. All of these results show that the water droplet on both the flat and the micro-pillar surface does not slide even at a 90° tilting angle, whereas the water droplet on the micro-grooved surface slides at 24° in the parallel direction. More importantly, the results from the sliding angle measurement are consistent with that from the contact angle hysteresis measurement.

4. Conclusions

This research studied the wetting characteristics of bare hydrophobic micro-patterned cyclic olefin copolymer surfaces machined by a one-step fabrication method, ultra-precision raster milling (UPRM), which is potentially applicable for mass production in plastic injection molding. Due to the shape edges induced by the numerical-controlled tool path of material removal process in the mechanical machining, the area fraction of the projected wet area on the micro-patterned surfaces can be defined accurately in the Cassie and Baxter model. This results in the large agreement of static contact angle predicted by the models with the measured static contact angle of water droplet from the experiment. By studying the droplet anisotropy, the interaction between droplet contact line and micro-patterned surfaces, it is found that the water droplet governed by the Cassie and Baxter regime is stabilized by the shape edges at the top asperities of micro-grooves and micro-pillars after its deposition on the sample surfaces. The experimental results from the contact angle hysteresis and sliding angle measurements show that the micro-directional grooved surface gives a good sliding performance which is important for the potential applications in microfluidic systems such as artificial self-cleaning surfaces for mass production in plastic injection molding.

Acknowledgements

The authors would like to express their sincere thanks to the Research Committee of The Hong Kong Polytechnic University, and The Innovation and Technology Fund of the Government of the Hong Kong Special Administrative Region of the People's of Republic of China for providing the financial support for this research work under the Project No. 4-RPLU and ITS/390/09 respectively.

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