Fabrication of biomimetic anisotropic crescent-shaped microstructured surfaces by laser shock imprinting

The design of the crescent-shaped microstructure was inspired by the biological structural characteristics of the surface of the internal slip zone of the Nepenthes leaf cage [12, 26]. The main parameters of the fabricated crescent-shaped microstructure unit on the surface of the mold are shown in Fig. 1(a), which consists of two circular arcs, R₁ and R₂, surrounded by two circular arcs: R₃ with the same radius tangent, where the distance between the two circles is L. The parameters of the crescent-shaped microstructure characteristic array are shown in Fig. 1(b), where X₀ is the spacing of adjacent crescents in the peer group, Y₀ is the spacing of adjacent rows, and P is the offset distance between adjacent rows.

Fig. 1.

In this investigation, the surface of the glass mold was selectively removed by the etching method. The specific plan was as follows: On the cleansed glass material, an adhesive (positive photoresist, AZ1500) was applied and then dried. The mask master was covered with the ;urface, and it was exposed to UV light (H94-30,SVC). The lighttransmitting part of the photoresist protective layer on its surface was removed. The this part removed was the crescent-shaped microstructure required by the study. It was then cleaned and dried. The lighttransmitting part of the adhesive protective layer was etched with hydrofluoric acid plus ammonium fluoride buffer solution (ratio 1:7), and the time required for corrosion was approximately 2 hours. The material was taken out of the solution and cleaned. The material was placed in a 100? thermostatic oven for 30 minutes so that the adhesive protective layer formed a firm film. The adhesive protective layer and coating layer were removed with alcohol. The mold was cut into the required single-piece size by the scribing machine.

The three molds fabricated are shown in Table 1. Taking mold 3 as an example, the fabricated 20 mm × 20 mm × 2 mm glass mold is shown in Fig. 1(c), with a 10 mm × 10 mm array of crescent-shaped microstructure grooves in the central region. The fabricated mold is shown in Fig. 1(d); the X₀ is 90.43 μm, the Y₀ is 60.46 μm, the L is 44.96 μm, and the P is 44.96 μm.

LSI uses a high-power, short-pulse laser to interact with a substance to generate a strong shock wave in a short period of time, and then the shock wave energy is used to shape the workpiece. The principle of LSI is shown in Fig. 2, where the focused high-energy density laser passes through the transparent confining layer and radiates on the surface of the ablative layer. The surface of the ablative layer material will absorb the laser energy and rapidly vaporize and ionize, forming a high-pressure plasma and simultaneously generating a strong shock wave, which is homogenized when passing to the bottom of the soft film and then acts on the metal sheet. When the pressure is much higher than the dynamic yield strength of the workpiece, the metal sheet instantly generates a high strain rate of plastic deformation and fits closely with the surface of the micromold, which can replicate the micro- and nanostructure of the mold surface [27]. From top to bottom, the LSI experimental platform consists of a constrained layer, an ablative layer, a flexible film, a workpiece, and a micromold. The materials selected are shown in Table 2. The constraint layer is utilized to increase the shock wave pressure and extend the duration of action. As the constraint layer, a polymethyl methacrylate (PMMA) plate with a thickness of 3 mm is used to guarantee light transmission. Aluminum foil with a thickness of 20 μm is used as the ablative layer to absorb the laser energy and protect the surface of the workpiece from thermal ablation effects. A soft polyurethane rubber film is chosen as the shock wave energy transmission medium so that the Gaussian-distributed laser shock wave energy is loaded more uniformly on the workpiece surface. A copper foil with a thickness of 10 μm was chosen as the workpiece. As the imprinting mold (the mold parameters are shown in Table 1), a crescent-shaped microstructure array was fabricated on the glass surface by etching. By regulating the processing parameters of LSI (as shown in Table 3), large-area crescent-shaped microstructure arrays were fabricated on the surface of copper foil workpieces. The laser in use was a Spitlight 2000 Nd:YAG nanosecond-pulsed laser from InnoLAS, Germany. As depicted in Fig. 3, the entire LSI system consists of a laser, a beam-focusing system, and a three-directional motion stage that facilitates movement in the X, Y, and Z directions. The laser beam is emitted from the laser emitter port, passes through the reflector and focusing mirror, and acts vertically on the workpiece surface.

Fig. 2.

Fig. 3.

A super-depth-of-field 3D microscope (VHX- 1000C, KEYENCE, Japan) was used for the measurement of 2D and 3D morphological characteristics of microstructures on the mold and workpiece surfaces. The video-based optical contact angle meter (OCAH200, Dataphysics, Germany) was used for the measurement of contact angle on the workpiece surfaces to detect the wettability of the workpiece surfaces. The morphology of the crescent-shaped microstructures fabricated by LSI was observed using a scanning electron microscope (SEM) (Hitachi, S-3400N, Japan).

The main parameters of the crescent-shaped microstructure used in the LSI process are as follows: X0 = 90 μm, Y0 = 60 μm, P = 0 μm and 45 μm. The LSI process parameters were 1020 mJ laser energy, a spot diameter of about 2 mm, 3 shock times, and a 0% lap rate. Figure 4 shows the surface of the workpiece after LSI. Figures 4(a) and (b) depict the surface of the imprinted workpiece with 0 μm and 45 μm offset distances, respectively. The imprinted workpiece surfaces with two different offset distances were neatly distributed in forming areas, and the crescent-shaped microstructure array in the imprinted area was formed without any breaks or incomplete or missing imprinting. As depicted in Figures 4(c) and (d), the individual spot imprint ranges of the two surfaces were measured with diameters of 2012.16 μm and 2021.38 μm, respectively, as expected. The morphology of the crescent-shaped microstructure is depicted in Figure 5. From Figures 5(a), (b), (d), (e), and (f), it is apparent that the edges of the individual crescent structure were fully formed with well-defined contour boundaries and complete surface replication after imprinting, and the dimensions of the individual crescent were measured as shown in Fig. 5(c), with R1 of approximately 30 μm and R2 of approximately 36 μm. Figures 6(a) and (b) illustrate the three-dimensional morphologies of the imprinted surfaces with two offset distances of 0 μm and 45 μm, respectively. When the same LSI-processing parameters were used, the crescent-shaped microstructures on the surface of the workpiece imprinted at the two offset distances were arranged neatly and regularly. The shape of the single microstructure resembles a crescent-shaped boss with a large bottom and a small top, and the height H was approximately 9.2 μm, which was conducive to the demolding of the workpiece after imprinting.

Fig. 4.

Fig. 5.

Fig. 6.

Three LSE (835 mJ, 1020 mJ, and 1200 mJ) and three laser shock numbers (1, 2, and 3 times) were used to fabricate the microstructure surfaces (The processing parameters of LSI are shown in Table 3). The three-dimensional morphologies of the workpiece surfaces fabricated by different LSI parameters are shown in Figure 7. From Figures 7(a), (b), and (c), it can be seen that, with the increase in LSE, the crescent-shaped microstructure morphology of the workpiece surface is increasingly close to the morphology of the mold surface; the top of the crescent shape is increasingly close to the bottom of the mold. When the LSE reaches 1020 mJ, the top of the replication degree basically reaches the optimum. When the LSE increases to 1200 mJ, the overall morphology does not change much. Comparison of Figs. 7(d), (e), and (f) shows that when the LSE is constant at 1020 mJ, the degree of plastic deformation of the workpiece increases with the increase of the number of shocks, and the degree of replication of the crescent-shaped microstructure on the workpiece surface increases, and the forming performance reaches its best when three shocks are applied.

Fig. 7.

Figure 8 shows the forming height of the surface microstructure with different LSI processing parameters. The height of the formed surface microstructure increases with increasing shock numbers at a constant LSE. For the same number of shocks, there is a positive relationship between the forming height and the LSE. As a result of the microstructure groove depth of the mold, the maximal forming height (approximately 9.1 μm) was reached when the LSE increased to 1020 mJ for three shocks. Further analysis reveals that the microstructure height change generated by the first shock will be significantly higher than the subsequent height change. For example, when the LSE is 1020 mJ, the initial shock generates a height change of 6.03 μm, and the second shock generates a change of only 1.63 μm. This is mainly caused by the technological characteristics of LSI. Only after the plasma shock wave generated by the ablative layer is transmitted to the surface of the soft film can it further act on the surface of the workpiece. As shown in Figure 9(a), before the first LSI, the workpiece had not been plastic deformed and had a high degree of contact with the surface of the soft film, so the efficiency of shock wave transfer was high and the resulting plastic deformation was large. As shown in Figure 9(b), before the second LSI, due to the plastic deformation of the workpiece, an air cavity is generated between the workpiece and the soft film, and the mutual contact degree is significantly reduced, so the efficiency of shock wave transmission is also significantly reduced; therefore, the height change of the second LSI is significantly reduced compared with the first LSI. It can be considered that the greater the degree of initial plastic deformation of the workpiece, the more obvious the phenomenon.

Fig. 8.

Fig. 9.

The heights of different positions of a single crescent-shaped microstructure fabricated by three shocks with 1020 mJ energy can be observed in Figure 10. The location of the center of symmetry is shown in Figure 10(a), and its contour line is shown in Figure 10(d) with a height of about 9.77 μm. Figure 10(b) shows the position near the right rounded corner, and its contour line is shown in Figure 10(e) with a height of about 8.79 μm. The location of the localized internal section of the structure is shown in Figure 10(c), with the contour line shown in Figure 10(f). which means that the height of the position at the center is higher than the height of the position near the right round corner. Analysis speculates that there are two reasons for this. On the one hand, the cavity size of the mold in the center position is wider than that of the position near the right corner of the lead. The principle of the schematic diagram is shown in Figure 11. The workpiece is extruded into the groove of the mold through shock energy. Under the same LSI-processing parameters, due to the influence of the size effect of the material itself, the wider the cavity size, the easier the workpiece flows into the cavity, and the better the reproduction effect on the mold. Another aspect is a result of the fact that the workpiece at the center of the symmetry position is formed with constraints mainly in two directions, whereas the workpiece near the right round corner position is constrained in approximately three directions. As shown in the schematic diagram of Figure 12, multidirection constraints will cause more limitations on material flow.

Fig. 10.

Fig. 11.

Fig. 12.

As LSE and the number of shocks increase, they will exceed the bearing limit of the ablation layer. As shown in Figure 13(a), when the LSE was 835 mJ and the number of shocks was three, the ablated layer had no holes. As depicted in Figure 13(c), when the LSE was 1200 mJ and the number of shocks was three, the ablated layer had obvious holes. It was indicated that part of the laser energy was not fully absorbed by the ablated layer, and the laser energy utilization was decreased. As shown in Figure 13(b), with a LSE of 1020 mJ and 3 shocks, the ablated layer exhibited only a few cracks. It was indicated that the ablated layer absorbed laser energy more efficiently to guarantee the surface-forming quality of the workpiece. With a LSE of 1020 mJ and 3 shocks, the workpiece surface crescent-shaped microstructure morphology replication degree basically reached the best replication. Therefore, the optimal LSI parameter for the subsequent wettability study was three times the shock of 1020 mJ energy.

Fig. 13.

Figure 14 shows the measurement results of the contact angle on the surface of the workpiece fabricated by LSE of 1020 mJ and 3 shocks. The contact angle in the raw material region was 75°, while the contact angle in the crescent-shaped microstructure region was 96.5°. The crescent-shaped microstructure considerably enhanced the hydrophobicity of the copper foil’s surface. Figure 15 depicts the measurement results of the contact angle on the surface of the workpiece fabricated with various LSI processing parameters, while Figure 15(a) depicts the effect of the number of shocks on the contact angle at LSE of 1020 mJ. As the number of shocks increases, the contact angle of the surface of the workpiece progressively increases. Figure 15(b) depicts the contact angle on the surface of the workpiece at LSE values of 835 mJ, 1020 mJ, and 1200 mJ, when the number of shocks is three. The figure demonstrates that as LSE increases, the contact angle increases, with the contact angle hardly changing once the energy exceeds 1020 mJ. As shown in Figures 8 and 15, it was discovered that the change law of the contact angle of the imprinted surface coincides with the change law of the height of the crescent-shaped microstructure.

Fig. 14.

Fig. 15.

Due to the asymmetry of the crescent structure shape and feature array in each direction, the fabricated workpiece surface wettability appears anisotropic. Observed from a parallel direction, the contact angle on both sides of the workpiece surface was nearly identical, as a result of the symmetry of the crescent structure shape and feature array in a parallel direction. When observed from the vertical direction, however, the contact angle of the outer arc direction of the crescent-shaped microstructure was always slightly smaller than that in the inner arc direction (i.e., the contact angle on the right side was smaller than the contact angle on the left side), as a result of the asymmetry of the crescent structure shape and feature array in the vertical direction. Figure 16 depicts the morphologies of water droplets in different directions on the surface of two crescent-shaped microstructure workpieces fabricated with an LSE of 1020 mJ, 3 shocks, and an offset distance of 45 μm. As depicted in Figure 16(a), the contact angles of water droplets observed from a parallel direction were identical: 99° on both sides. As depicted in Figure 16(b), the contact angles of the water droplets observed from the perpendicular direction were as follows: The contact angle on the left side was 100° and on the right side was 96.5°, and the difference between the contact angles on both sides was 3.5°. This indicates that the fabricated crescent-shaped microstructure workpiece has a surface with an obvious anisotropy.

Fig. 16.

While the shape of the crescent-shaped microstructure remained unchanged, the influence of the offset distance (P) of the microstructure feature arrangement on the anisotropy of the surface was investigated and further analyzed. The molds with offset distances of 0 μm, 30 μm, and 45 μm were selected, and three kinds of crescent-shaped microstructured workpiece surfaces were fabricated using LSI with the process parameters of LSE of 1020 mJ and the number of shocks of three times. The effect of different offset distances in the vertical direction on the contact angle difference between water droplets on both sides of the crescent-shaped microstructured workpiece surface is depicted in Figure 17. It can be seen from the figure that when the offset distance is 0 μm, the contact angle difference is relatively small and the anisotropy is at its weakest, and the contact angle difference at this time is mainly due to the asymmetry of the crescent-shaped microstructure shape. As the offset distance increases, the contact angle difference also increases, and when the offset distance reaches 45 μm, the contact angle difference between the two sides of the water droplet is at a maximum, approximately 3.5°. Since the design parameter of the vertical alignment spacing of the microstructure features is 90 μm, the superposition of crescent-shaped microstructure shape and microstructure feature alignment asymmetry has the greatest influence, resulting in the best anisotropy on the workpiece surface.

Fig. 17.

3.2.1.

Morphological analysis of droplet solid–liquid contact lines

The structural asymmetry and arrangement of the crescent-shaped microstructures on the surface of the workpiece lead to the generation of asymmetric droplet-spreading resistance, resulting in a change in the shape of solid–liquid contact lines on the fabricated surface of the workpiece that is different from that of the circular solid–liquid contact lines on the anisotropic surface, and a solid–liquid contact line resembling a straight line is generated. The suspended droplet on the syringe was slowly put in contact with the microstructure surface and deposited (water, volume of about 1 μL), and the morphology of the droplet was observed. The whole residual droplet was photographed as shown in Figure 18(a), where the solid–liquid contact lines on the left and right sides of the droplet were similar to arcs, and the solidliquid contact line at the bottom of the droplet (the droplet side in the direction of the bottom of the arcs) was an overall straight–line-like shape. The enlarged image is shown in Figure 18(b). The analysis speculates that it may be due to the fact that the barriers against which the droplets spread towards the bottom of the arc are larger compared to the other directions, thus the droplets are more difficult to cross as compared to the other directions, which is reflected in the fact that the solid-iquid contact line is restricted to an overall similarly straight shape.

Fig. 18.

During droplet motion, the direction of liquid movement depends on the asymmetric resistance generated by the special microstructure [28]. According to Jing et al. [29], the cause of anisotropic wettability can be analyzed by establishing the equation of resistance to droplet motion on a plane. Therefore, a single crescent-shaped microstructure is taken as the object of study and the following equation is established:

1α=arcsin(l2R)$$\alpha = \arcsin \left( {{l \over {2R}}} \right)$$ 2La=2απR180$${L_a} = {{2\alpha \pi R} \over {180}}$$ 3f=γ(cosθ−cosθ1)$$f = \gamma \left( {\cos \theta - \cos {\theta _1}} \right)$$ 4F=f⋅La$$F = f \cdot {L_a}$$ 5Ft=f⋅La2=f⋅2α2πR2180$${F_{\rm{t}}} = f \cdot {L_{a2}} = f \cdot {{2{\alpha _2}\pi {R_2}} \over {180}}$$ 6Fb=f⋅La1=f⋅2α1πR1180$${F_{\rm{b}}} = f \cdot {L_{a1}} = f \cdot {{2{\alpha _1}\pi {R_1}} \over {180}}$$

As shown in Figure 19(a), the solid–liquid contact line of a single crescent-shaped microstructure on the surface of the workpiece is extracted, and the solid-liquid contact line generated when the droplet spreads in the direction of the bottom of the arc is the outer arc L_a1, and the solid–liquid contact line generated when the droplet spreads in the direction of the top of the arc is the inner arc L_a2. Figure 19(b) shows the mathematical model of the solid–liquid contact line for the part of the crescent-shaped microstructure. Measurement of the fabricated crescent-shaped microstructure yields an outer arc radius R₁ of about 30 μm and an inner arc radius R₂ of about 36 μm, which are calculated by substituting into Equations (1) and (2) to yield an outer arc with a longer solid–liquid contact line. According to Equations (3) and (4), the motion resistance F is related to the solid–liquid contact line L_a as well as the motion resistance per meter of the droplet f. From Equations (5) and (6), the resistance to spreading towards the bottom of the arc, F_b, is greater than the resistance to spreading in the direction towards the top of the arc, F_t. Therefore, the anisotropic wettability in the direction of the top and bottom of the arc are formed, which causes the solid–liquid contact line to take on a special shape.

Fig. 19.

According to the study above, the increase in the surface contact angle is positively correlated with the increase in the height of the crescent-shaped microstructure. Therefore, different LSI-processing parameters can be used to fabricate surfaces with different wettability, and a crescent-shaped microstructure gradient surface is designed for the directional spreading of droplets on gradient wetting surfaces [30]; i.e., water droplets will move and spread to the side with high wettability because of the surface energy gradient. Since the anisotropy of the crescent-shaped microstructure causes anisotropic wetting of the workpiece surface, the array feature arrangement with the greatest anisotropy (Mold 3 in Table 1) is used as the mold for the fabrication of the microstructured surface. Surfaces with a wettability gradient are fabricated by controlling the LSI-processing parameters so that there is a significant difference in the wettability of the two zones. Figure 20 depicts the designed wettability gradient surface schematically. The left side (Fig. 20a) is a low wettability surface, while the right side (Fig. 20b) is a high wettability surface. Both sides constitute a wettability gradient, and the direction of the top of the crescent is also the direction of increasing wettability. The blue circle is the theoretical forming area of the crescent-shaped microstructured surface.

Fig. 20.

Water droplets are injected at the junction of two zones processed with different LSI-processing parameters, with the left side having low wettability and the right side having high wettability. The theoretical spreading schematic for a liquid is shown in Figure 21, where the red dashed line indicates the location of water droplet injection, the dark brown circle on the left side represents the low wettability area, and the light brown circle on the right side represents the high wettability area. The spreading of droplets in the low wettability zone on the left will be limited, causing the droplets to move as a whole toward the high wettability zone on the right.

Fig. 21.

Based on the fact that the crescent-shaped microstructured surface is anisotropic, the droplet boundary on the bottom side of the arc generates a clear solid–liquid contact line that is similar overall to a straight line. Flow channels can be designed and fabricated by changing the distribution of crescent-shaped microstructure areas to limit the continuous spreading of liquid droplets to both sides. In Figure 23(a), a schematic of the distribution of the planar microstructure area of the type-I flow channel is depicted. The black forked arrow indicates the direction to restrict droplet movement; the blue section is the theoretical area in which droplets spread; and the brown circles on both sides represent the microstructure array zone fabricated by LSI. Figure 23(b) shows a partially magnified image of the microstructured zone in 18(a), with the crescent-shaped microstructures arrayed in the direction of the outer arc toward the I-type flow channel. A mold with the most anisotropic array structure parameters (Mold 3 in Table 1) and optimal LSI-processing parameters (LSE of 1020 mJ, 3 shocks) was used to fabricate the type-I flow channels. As shown in Figure 22, the distribution of the crescent-shaped microstructure zone was controlled by changing the position of the mold and the workpiece; the numbers (1, 2, 3, …) are the order in which the laser beams are scanned, and the red arrows are the moving path of the laser beam. The fabricated type-I flow channel is depicted in Figure 23(c). The inner and outer layers of the flow channel consist of several microstructure areas, and only the effect of the inner layer microstructure areas on droplet spreading was investigated in this study. The circular crescent-shaped microstructure-forming area was regularly distributed on both sides of the flow channel, which together formed a type-I structureless zone with a width of approximately 1 mm. The edges of the flow channel area were regular serrated-like, the adjacent serrated tooth spacing was approximately 1.8 mm, and the maximum tooth base spacing on both sides of the flow channel was approximately 2.3 mm. The morphology of the inner part of the type-I flow channel (in Fig. 23c) was observed. As depicted in Figure 23(d), the crescent-shaped microstructure on the inside of the type-I flow channel had the outer arc towards the flow channel side and the inner arc back to the flow channel side. The forming quality of the crescent-shaped microstructure near the inside of the flow channel was poor compared to the outside, and the microstructures at the edges of the flow channel in different spot areas were not connected together. The edges appear to be jaggedly separated, resulting in a jagged shape in the structureless area of the flow channel.

Fig. 22.

Fig. 23.

The same liquid (water, volume 18 μL) was dropped into the flow channel and raw material area by the method of slowly dropping it several times in the direction of the flow channel and then causing the droplets to converge. When the droplets converged, the morphology of the liquid in the two areas was observed. As shown in Figure 24(a), as the droplets were continuously dropped, the droplet expansion range in the microstructure area on both sides inside the flow channel area was significantly smaller than that in the raw material area, and the droplets in the raw material area always showed an oval shape due to the influence of the surface tension of the material. The measured long axis was approximately 5.4 mm, and the measured short axis was approximately 4.8 mm, whereas the droplets inside the flow channel area stopped expanding and spreading after expanding to a certain position on both sides of the flow channel, and the phenomenon of liquid droplet being fixed occurred. The final shape is a strip; the length in the flow channel direction was approximately 7 mm, and the length perpendicular to the flow channel direction was approximately 3 mm. Re-drop the droplet in the runner area with the method described above. The liquid–solid contact edge in the microstructure area inside the flow channel was observed as shown in Figure 24(b), and both the upper and lower edges of the droplet inside the flow channel area were affected by the crescent-shaped microstructure, which generated solid–liquid contact lines that approximate straight lines. The fabricated flow channel realized the restriction of liquid droplets spreading on the surface of the workpiece.

Fig. 24.

The solid–liquid contact lines on both sides of the flow channel were discovered in the crescent-shaped microstructure region outside the central serrated region. As depicted in Figures 25(a) and (b), the blue line represents the boundary of the serrated region, and the red line represents the location where the fixed contact line was generated. The location where the liquid generated the fixed contact line was outside the serrated region. This is due to the volume of the water droplets used in this study and the process characteristics of LSI. The continuous increase in droplet volume on the surface of the workpiece per unit area also increases the pressure on the crescent-shaped microstructure on both sides of the solid–liquid contact line. When the resistance limit of the crescent-shaped microstructure is exceeded, the original fixed contact line will be broken and spread outward to form a new fixed position. As shown in the schematic diagram in Figure 25(c), since the laser output is a Gaussian beam (which generates a Gaussian distribution of LSE), the shock wave generated in the ablation layer will also exhibit a Gaussian distribution; the shade of color in the brown circular area represents the size of the shock wave energy; the darker the color, the higher the energy. The black circle represents the area with a more effective forming result. Despite the use of a soft rubber film to homogenize the shock energy, there is also an uneven phenomenon of forming pressure (high in the center and low at the edges) after a singlepoint shock. As a consequence, the microstructureforming result near the edge of the spot is relatively poor compared to that in the center of the spot (the formed microstructure has a lower height and does not sufficiently conform to the inner surface of the mold), and its wettability and anisotropy are also poor. At the same time, because the laser beam output is a circular spot, the continuous crescent-shaped microstructure is absent in the serrated area on both sides of the formed flow channel, and the region as a whole is less effective in limiting droplet spreading, whereas the middle area of the spot has a good lap, and the well-formed crescent-shaped microstructure has good continuity and is connected together, the effect of a good spreading limitation when the liquid spreads to this area.

Fig. 25.