Patent Publication Number: US-2021165330-A1

Title: Microstructure patterns

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/767,990, filed Apr. 12, 2018, which is a 35 U.S.C. § 371 national stage application of PCT International Application No. PCT/AU2016/050960, filed on Oct. 13, 2016, which claims the benefit of U.S. Provisional Patent Application No. 62/240,708, filed Oct. 13, 2015. The contents of the above patent application are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to a method and a system for patterning a microstructure on a surface. More particularly, the present disclosure relates to patterning a microstructure on an exterior surface. In one arrangement, the present invention provides a microstructure pattern on a top coat on an exterior surface of a vehicle. 
     BACKGROUND 
     The fuel consumption by modern aircraft depends significantly upon the drag experienced by the aircraft. Similar considerations apply in relation to boats and automobiles. It has been known for some time that the drag of an aerodynamic surface can be reduced by creating a microstructure pattern on the surface. 
     Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant and/or combined with other pieces of prior art by a person skilled in the art. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention there is disclosed a method of providing a microstructure pattern on an exterior surface of a vehicle, said method comprising the steps of:
         applying a layer of photocurable material to said exterior surface, said photocurable material including a photoinitiator;   selectively irradiating said photocurable material to activate said photoinitiator in only those regions of the photocurable material layer irradiated; and   removing either the un-irradiated photocurable material or the irradiated photocurable material,   wherein both the applying and irradiating steps do not involve a mask coming into contact with said photocurable material layer.       

     Preferably the photocurable material is a photopolymer. 
     In accordance with a second aspect of the present invention there is disclosed a method of providing a microstructure pattern on an exterior surface, the method comprising the steps of:
         applying a layer of photocurable material to the exterior surface;   irradiating the photocurable material with radiation including a predetermined irradiation intensity profile to initiate curing of the irradiated photocurable material, the curing causing a curing depth profile across the layer of the photocurable material corresponding to the selected intensity profile; and   removing uncured photocurable material to form the microstructure pattern.       

     In accordance with further aspects of the present disclosure, corresponding systems for providing a microstructure pattern on an exterior surface are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Arrangements of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic representation of a photopolymer before and after irradiation; 
         FIG. 2  is a schematic perspective view of a prior art rolling photolithography apparatus used in a continuous process to manufacture a microstructure pattern and in which a mask is in contact with the photopolymer; 
         FIG. 3  is a transverse cross-sectional view through the cylinder of the apparatus of  FIG. 2 ; 
         FIG. 4  is a schematic side elevation of a second prior art technique which may be termed the Fraunhofer technique and in which a web microstructure former or mould comes in contact with the photopolymer; 
         FIG. 5  is an enlarged view showing some details of the arrangement of  FIG. 4 ; 
         FIG. 6  is a schematic cross-sectional view of a roller apparatus in accordance of an arrangement of the present disclosure in which a mask comes into close proximity to, but not contact with, the photopolymer; 
         FIG. 7  is an enlargement of a portion of  FIG. 6  showing in detail the components thereof; 
         FIG. 8  is a view similar to  FIG. 6  but illustrating an alternative arrangement in which a predetermined intensity profile is provided by means of interference of two beams generated by a beam splitter; 
         FIG. 9  is an enlarged view of the central portion of the apparatus of  FIG. 8 ; and 
         FIG. 10  is a schematic illustration of a diffraction grating illustrating the interference pattern created using such a grating; 
         FIG. 11A  is a flow chart of an example of a method of providing a microstructure pattern on an exterior surface; 
         FIG. 11B  illustrates side views of outputs of steps of the described method illustrated in  FIG. 11A ; 
         FIG. 11C  illustrates top views of outputs of steps of the described method illustrated in  FIG. 11A ; 
         FIG. 12  illustrates an arrangement of a system for carrying out a step of the method illustrated in  FIG. 11A ; 
         FIGS. 13A to 13C  illustrate snapshots of irradiation of a layer of photocurable material by the system illustrated in  FIG. 12 . 
         FIG. 14A  illustrates another arrangement of a system for carrying out the method illustrated in  FIG. 11A ; 
         FIG. 14B  illustrates a snapshot of irradiation of a layer of photocurable material by the system illustrated in  FIG. 14A ; 
         FIG. 14C  illustrates yet another arrangement of a system for carrying out the method illustrated in  FIG. 11A ; 
         FIGS. 15A-15E  illustrate examples of microstructure patterns provided by the present disclosure; and 
         FIGS. 16A and 16B  illustrate examples of post-processing steps applicable to the method of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to a technique in providing a microstructure pattern on an exterior surface, such as on the top coat of a vehicle, such as an aircraft, a boat and an automobile, which travels through a fluid such as air or water. 
     Photocurable materials such as photopolymers are well known from photolithographic techniques developed for computer microchip fabrication and, as illustrated schematically in  FIG. 1 , the photopolymer  1  consists of a mixture of smaller molecules (monomers  2  and oligomers  3 ) and a photoinitiator  4 . 
     After exposure to ultraviolet light  6 , or radiation, normally via a mask, the photoinitiator catalyses a polymerization reaction between the monomers  2  and the oligomers  3  causing them to cross-link up into larger network polymer molecules and thereby form the cured polymer. These network polymers change their chemical and structural properties. So-called “negative photopolymers” become insoluble and stronger than the unexposed photopolymer. However, so-called “positive photopolymers” become soluble and thus weaker than the unexposed photopolymer. 
     Microstructures can thus be made by applying a thin layer of photopolymer to a substrate and exposing it to UV light or radiation through a photomask. Either the unexposed negative photopolymer is removed by use of a developer liquid which washes away the unexposed photopolymer, thereby leaving the exposed photopolymer in the desired pattern, or the exposed positive photopolymer is removed. 
     A liquid etchant can then be applied which attacks the substrate but not the remaining photopolymer. Consequently, when the remaining photopolymer is removed, the desired microstructure is created etched into the substrate. Other etching methods such as by means of a plasma, are also able to be used. 
     Photolithography Techniques 
     This general photolithography technique has been used in rolling mask photolithography in a continuous process as schematically illustrated in  FIGS. 2 and 3 . Here liquid photopolymer is applied via nozzles  10  to a substrate  11 . A cylindrical rolling mask  12  is rolled over the photopolymer and contains an internal coaxial source  13  of UV radiation. Downstream of the rolling mask  12  are nozzles  15  for the developer and nozzles  16  for the rinse. 
     As seen in  FIG. 3 , UV radiation from the source  13  passes through the mask  12  which is in contact with the photopolymer on the substrate  11  thereby forming the abovementioned photo polymerization reaction. The polymer coated substrate  11  then passes under the nozzles  15  and  16  to respectively remove the unexposed photopolymer from those portions of the substrate not covered by an exposed photopolymer and rinse the substrate  11 . 
     An alternative process is illustrated in  FIGS. 4 and 5 . In this Fraunhofer method the microstructure is formed out of the photopolymer and left on the aircraft surface rather than being etched into the aircraft surface, or substrate, as is the case in the prior art arrangement of  FIGS. 2 and 3 . In the arrangement of  FIGS. 4 and 5 , a UV transparent web  22  has a negative of the desired microstructure formed on its outside surface. The web  22  is preferably formed from silicone film and is transparent to UV radiation emitted from a UV lamp  21 . The web  22  passes over a pair of flexible rollers  23  and a guide roller  25 . A dosing unit  24  takes the form of a tank  30  and a pipe  31  which permits a liquid coating  26  to be applied to the web and formed from the liquid contained in the tank  30 . The liquid coating  26  is then applied to the upper surface of the substrate  27  by the rolling motion of the web  22  over the rollers  3 ,  5 . 
     As indicated in  FIG. 5 , the web  22  has a negative of the desired pattern and thus forms the photopolymer  32  on the substrate  27  into that desired pattern. The UV radiation  33  from the UV lamp  21  passes through the web  22  and sets the photopolymer  32  into the desired pattern formed by the cured photopolymer  28 . Thus, as the apparatus moves relative to the substrate  27  in the direction indicated by arrow  29 , so the cured photopolymer  28  in the desired pattern is formed on the substrate  27 . 
     In this method the rolling mask matrix material requires a very low surface energy and a Shore hardness within a specific narrow range. In addition, the liquid coating  26  must adhere to the substrate  27  after exposure yet not run or otherwise change shape after the web  22  is removed. Furthermore, the web  22  is expensive to produce and degrades through the rolling contact process. 
     Mask-Based Arrangement 
     Turning now to  FIGS. 6 and 7 , an arrangement of the present disclosure is described. The apparatus takes the form of a hood or shroud  41  which covers the apparatus and protects it from ambient UV light. Within the shroud are a pair of rollers  42  which permit the apparatus to move over a substrate  43 . 
     In an arrangement generally similar to that of  FIG. 2 , an array of nozzles  45  apply polymer to the substrate  43 , a further array of nozzles  46  applies liquid developer and a still further array of nozzles  47  applies a liquid rinse. Between the nozzles  45  and  46  is a rolling cylindrical mask  49  which contains a UV light source  50 . In an alternative arrangement, the mask may be substantially planar and is translated above and along said exterior surface. A skilled person would appreciate that description hereinafter of a cylindrical mask, with minor modifications, may be applicable to a substantially planar mask. 
     As best seen in  FIG. 7 , the mask  49  does not come into contact with the photopolymer  44  but is instead spaced therefrom by a small gap  51  of approximately 10-100 centimetres. 
     As schematically illustrated in  FIG. 6 , those portions of the photopolymer  44  which are exposed to the UV radiation from source  50  remain adhered to the substrate  43  after passing under the developer nozzles  46  and rinse nozzles  47 . The present arrangement, which utilises proximity printing techniques of computer microchip photolithography, can achieve a resolution down to 1-2 microns which is more than sufficient for microstructures which reduce aerodynamic, such as skin friction drag. The described arrangement allows for different photopolymer/developer combinations without the strict requirements for mask contact printing as described above in relation to  FIGS. 4 and 5 . In addition, different cylindrical masks  49  can be easily substituted to allow different microstructure arrangements to be applied, for example, to different areas of the exterior of a single aircraft. 
     It is also possible to use the arrangement of  FIGS. 6 and 7  so as to form the microstructure by etching the substrate  43 . This can be done by using additional etching nozzles, or by immersing an entire panel in the etching liquid. 
     Maskless Arrangement 
     In accordance with a further arrangement of the present disclosure, as illustrated in  FIGS. 8 and 9 , a maskless system can be created by use of interference lithography. Interference lithography allows for continuous patterning of regular arrays by setting up an interference pattern between two coherent light, or radiation, sources. The minimum spacing between features is equal to approximately half the wavelength which corresponds to a minimum spacing of approximately 0.2 microns for UV radiation. As indicated in  FIG. 8 , the apparatus of  FIG. 6  is modified by the removal of the cylindrical mask  49  and light source  50  and the provision instead of a UV laser  61 , a spatial filter  62 , a beam splitter  63  and a pair of mirrors  64 . In this arrangement, the wavelength for the UV laser is 364 nanometers. The mirrors  64  are moveable relative to the substrate  43  so as to increase or decrease the angle θ. This adjusts the spacing between the pattern lines generated by the interference arrangement. 
     As before, the present arrangement can be used to form etched patterns into the substrate  24  by the provision of additional etching nozzles. 
     Turning now to  FIG. 10 , the arrangement of  FIGS. 8 and 9  can be further modified so that instead of using beam splitting techniques, a diffraction grating  71  (e.g. in the form of a phase mask) is utilised instead. The diffraction grating  71  is uniformly illuminated from a UV source (not illustrated in  FIG. 10 ) so as to thereby again form an interference pattern on the substrate  43 . Under this arrangement the spacing pattern is not tunable but is instead determined by the construction of the diffraction grating. 
     Single-Exposure Arrangement 
     Some existing photolithographic arrangements require multiple-exposure to create a desirable microstructure pattern layer by layer (e.g. by multiple-exposure) across a surface. Described herein is a method and system for providing a microstructure pattern on an exterior surface that provides a microstructure pattern with a selected spatial profile without the need for multiple-exposure. 
     As illustrated in  FIG. 11A , the described method  1100  comprises the step  1102  of applying a layer of photocurable material to the exterior surface, the step  1104  of irradiating the photocurable material with radiation including a predetermined irradiation intensity profile, and the step  1106  of removing uncured photocurable material to form the microstructure pattern. The radiation initiates curing of the irradiated photocurable material, causing a curing depth profile across the layer of the photocurable material corresponding to the selected intensity profile. The correspondence may include a linear or a non-linear relationship between the selected intensity profile and the curing depth profile. The removing step  1106  of may occur after completion of the curing. 
       FIGS. 11B and 11C  illustrate schematically a side view  1150  and a top view  1160 , respectively, of an example of the intermediate or final output after each of steps  1102 ,  1104  and  1106  of the described method  1100 . In this example, the layer of photocurable material is a UV-curable or near-UV-curable coating  1152 , which upon curing adheres to the exterior surface. The coating  1152  may be designed for specific use, such as up to military specifications including the MIL-PRF-85285 specifications. In another instance, the coating  1152  is primer-surfacer Cromax  3130 S. In this example, the exterior surface is a substrate  1154 , such as the top coat of a vehicle. In the example illustrated in  FIGS. 11B and 11C , the predetermined irradiation intensity profile is a sawtooth irradiation intensity profile  1156 . In this example, where the intensity-to-curing-depth correspondence is a linear relationship, the resulting microstructure pattern includes a sawtooth riblet geometry  1160 . In another example, where the intensity-to-curing-depth correspondence is a non-linear relationship, the resulting microstructure pattern includes a scalloped riblet geometry. 
     Microstructure Patterning Systems 
       FIG. 12  illustrates an arrangement of a microstructure patterning system  1200  configured to carry out the irradiating step  1104  in the described method  1100 . In this arrangement, the step  1102  of applying the coating  1152  to the substrate  1154  (which has already taken place) and the step  1106  of removing the uncured photocurable material (which has not yet taken place) are carried out separately and not by the system  1200 . 
     The system  1200  includes a radiation source  1202 . The radiation source  1202  may be a near-UV light source. In one example, the near-UV light source is a 405 nm laser diode with power output of up to 50 mW. The laser diode behaves as a point-like source producing in phase incident light. This wavelength allows photomasks to be made from glass rather than quartz, which would otherwise be necessary for UV wavelengths. In another system, other wavelengths may be used. The system  1200  includes a radiation modifier  1203  to modify the radiation to produce desirable irradiation to the layer of photocurable material. In one arrangement, the radiation modifier  1203  includes an amplitude mask  1204  and/or phase mask  1206 . To achieve a predetermined irradiation intensity profile, the radiation is passed through an amplitude mask and/or a phase mask associated with the predetermined irradiation intensity profile. In case of an amplitude mask  1204 , it may be a gray-scale mask, having different transparency or attenuation based on position on the mask. In case of a phase mask  1206 , it may be in a form of a one-dimensional diffraction grating providing an interference pattern  1209  upon illumination. The predetermined irradiation intensity profile in the presence of bottom-up curing (see more description below) allows creation of a microstructure pattern without the need for multiple-exposure. 
     In this arrangement, the irradiation intensity profile has variations along a first dimension  1211 , causing a curing depth profile with variations also along the first dimension  1211 . The radiation modifier  1203  may include a shutter  1208  to limit the exposed area of the layer of the photocurable material  1152  along the first dimension  1211 . The radiation modifier  1203  may also include a photoresist mask  1214  to limit the exposure along a second dimension  1212 , substantially orthogonal to the first dimension  1211 . The radiation source  1202  and/or the radiation modifier  1203  are supported by a support rig  1210 . The support rig  1210  is configured to displace, such as raising and lowering, the supported components to change the distance from the radiation modifier  1203  to the layer of the photocurable material  1152 . The support rig  1210  is also configured to displace, such as translating along the second dimension  1212 , the radiation source  1202  and the radiation modifier  1203  to irradiate a different part of the layer of photocurable material  1152 . The displacement of the radiation modifier  1203  allows exposure of an area of the layer of photocurable material  1152  larger than the aperture of the radiation modifier  1203 . 
       FIGS. 13A to 13C  illustrate snapshots of irradiation of a layer of photocurable material  1152  by the system  1200  with displacement. For example, as illustrated in  FIG. 13A , where the photoresist mask  1214  and/or the shutter  1208  limit the radiation exposure to a substantially linear dimension, the radiation source  1202  and the radiation modifier  1203  are translated in a continuous motion along the second dimension  1212  to achieve exposure area larger than the aperture of the radiation modifier  1203 . As another example, as illustrated in  FIG. 13B , where the photoresist mask  1214  and/or the shutter  1208  allow more radiation exposure along the second dimension  1212 , the radiation source  1202  and the radiator modifier  1203  are translated in a shuttered manner (i.e. translate-expose-shutter in repeated cycles) along the second dimension  1212  to achieve exposure area larger than the aperture of the radiation modifier  1203 . In either example, the periodicity in the curing depth profile along the first dimension  1211 , with or without the support rig translation along the second dimension  1212 , results in the formation of one or more of the following microstructure patterns: a sawtooth riblet geometry ( FIG. 15A ), a scalloped riblet geometry ( FIG. 15B ) and a blade riblet geometry ( FIG. 15C ). Where the exterior surface is part of a vehicle&#39;s exterior surface, these geometries are known to reduce the parasitic drag, such as skin friction drag, experienced by the vehicle as the vehicle moves relative to a fluid, such as air or water. In essence, the microstructure patterns of  FIGS. 15A to 15C  have the effect of delaying or reducing separation of a fluid boundary layer adjacent the exterior patterned surface. The relatively delayed or reduced separation of the fluid boundary layer results in reduced skin friction drag. Advantageously, by reducing parasitic drag, the vehicle may, for example, experience increased fuel efficiency. A person skilled in the art will appreciate that a number of different non-illustrated microstructure patterns may have the same effects as those shown in  FIGS. 15A to 15C . 
       FIG. 14A  illustrates another arrangement of a microstructure patterning system  1400 . Unlike the system  1200 , the system  1400  is configured to undertake all of steps  1102 ,  1104  and  1106 . The system  1400  includes a photocurable coating applicator  1402  for applying a photocurable coating, an irradiator  1404  for irradiating the photocurable material with radiation  1403  including a predetermined irradiation intensity profile, and a remover  1406  for removing uncured photocurable material to form the microstructure pattern. The irradiator  1404  may include a radiation source  1202  and a radiation modifier  1203 . The remover  1406  includes a develop applicator  1406   a  for applying a developer  1407   a  to facilitate separation of the uncured photocurable material from the cured photocurable material. The remover  1406  also includes a rinse applicator  1406   b  for applying a rinsing agent  1407   b  to rinse off the uncured photocurable material. The choice of the developer  1407   a  depends on the photocurable material used. For instance, the developer can be a mineral alcohol for UV-curable coatings. In some arrangement, physical removal with compressed air may be possible for some photocurable materials. 
     In this arrangement, the system  1400  includes an enclosure  1408  to enclose the photocurable coating applicator  1402 , irradiator  1404  and the remover  1406  positioned in this order. Further, the system  1400  includes two wheels, a front wheel  1410   a  and a rear wheel  1410   b , to roll on the substrate  1154  (with or without the photocurable material  1152 ). In use, the system  1400  can be rolled in the direction from the rear wheel  1410   b  to the front wheel  1410   a . The front wheel  1410   a  is placed near the photocurable coating applicator  1402 , which carries out the first step (step  1102 ) of the described method  1100 , whereas the rear wheel  1410   b  is placed near the remover  1406 , which carries out the last step (step  1106 ) of the described method  1100 . 
       FIG. 14B  illustrates a snapshot in carrying out the method  1100  by the system  1400  when rolled on an aircraft surface  1412 . The photocurable coating applicator  1402  applies a photocurable coating  1414  to the aircraft surface  1412 . Similar to the illustration in  FIG. 12A , the photoresist mask  1214  and/or the shutter  1208  in the irradiator  1404  limit the radiation exposure to a substantially linear dimension with an interference pattern  1209 . As the system  1400  is rolled along the dimension  1212 , the photocurable material upon irradiation becomes cured photocurable material  1416  over time and exhibits a curing depth profile. The remover  1407  then develops and rinses to remove uncured photocurable material  1417  to form a microstructure pattern  1418 . 
       FIG. 14C  illustrates a similar arrangement of a microstructure pattern system  1450  to the system  1400  but without any wheels. In this arrangement, to achieve an exposure area larger than the aperture of the radiation modifier, the system  1450  includes a robotic arm  1452  which supports the enclosure  1408  of the system  1400  (less the wheels  1410   a  and  1410   b ) and moves in a shuttered (i.e. translate-expose-shutter) or a continuous manner. 
     In the arrangement of  FIG. 12 , the radiation modifier  1203  does not provide any variations in the irradiation intensity profile in the second dimension  1212 . This permits a periodic curing depth profile with periodicity (and hence periodic patterning of microstructures) in the first dimension  1211  across the layer of irradiated photocurable material, as well as a substantially non-periodic profile in the second dimension  1212 . For example, the support rig  1210  may be configured to translate the radiation source  1202  and the radiator modifier  1203 , relative to the substrate  1154  at a constant speed, along the second dimension  1212  to provide a substantially constant curing depth profile in the second dimension  1212 . In another arrangement, the translation speed may be controlled in a variable fashion to provide a non-constant curing depth profile in the second dimension  1212 , with the varying translation speed corresponding to the non-constant profile in the second dimension  1212 . Lower translation speeds generally correspond to larger curing depths and vice versa. For example, a translation speed in a sawtooth fashion may yield an inverse sawtooth curing depth profile in the second dimension  1212 . In yet another arrangement, the translation speed may be constant but the overall intensity (with or without the intensity profile) may be controlled in a variable fashion to provide a non-constant curing depth profile in the second dimension  1212 , with the varying overall intensity corresponding to the non-constant profile in the second dimension  1212 . Lower overall intensities generally correspond to small curing depths and vice versa. For example, an overall intensity varied in a sawtooth fashion may yield a sawtooth curing depth profile in the second dimension  1212 . As a skilled person would appreciate that sawtooth or inverse sawtooth profiles are illustrative only, the non-constant curing depth profile can result in a variety of non-constant microstructure pattern a having variation along the second dimension. In one example, the height variation can manifest in a tapered riblet geometry, where each riblet includes a sawtooth profile in one dimension and a ramp-up portion, plateau portion and a ramp-down portion in the orthogonal dimension. Other examples can be found in, for instance, U.S. Pat. No. 6,345,791. 
     In an alternative arrangement, the radiation modifier  1203  may include another one-dimensional amplitude or phase mask (not shown) or may replace the one-dimensional amplitude or phase mask with a two-dimensional amplitude or phase mask, to provide variations in the irradiation intensity profile along the second dimension  1212 , causing a curing depth profile with variations also along the second dimension  1212 . In this arrangement, the radiation source  1202  and the radiator modifier  1203  are translated in a shuttered manner, as illustrated in  FIG. 13C , to achieve an exposure area larger than the aperture of the radiation modifier  1203 . The periodicity in the curing depth profile along the first dimension  1211  and the second dimension  1212 , with or without the support rig translation along the second dimension  1212 , results in the formation of one or more of the following microstructure patterns: a lotus leaf geometry ( FIG. 15D ) and a superomniphobic geometry ( FIG. 15E ). Some of these geometries have a self-cleaning property to reduce the cleaning or maintenance requirements of, for example, an aircraft. 
     In the geometries shown in  FIGS. 15A to 15E , the feature size of such geometries can be down to approximately 10 microns and heights up to approximately 100 microns. 
     Bottom-Up Curing 
     In one arrangement, the curing includes bottom-up curing. With reference to the example illustrated in  FIGS. 11B and 11C , bottom-up curing refers to a curing process which begins at a first side of the layer of the photocurable material proximal to the exterior surface (i.e. the bottom side  1162 ), and continues towards an opposed, second side distal from the exterior surface (i.e. the top side  1164 ). In the absence of bottom-up curing, the curing may be instantaneous or near instantaneous upon irradiation. Conversely, bottom-up curing allows curing to spatially progress over time from the bottom side  1162  to the top side  1164 . The bottom-up curing continues to progress until any one of the following occurs: the uncured photocurable material is removed, the layer of the photocurable material is fully cured, or the curing is inhibited from progressing any further (see further description below). The maximum height of the microstructure pattern can therefore be controlled by one or more of following: the thickness of the layer of the photocurable material, the timing of removing step  1106 , and the extent of inhibited curing. 
     The bottom-up curing gives rise to areas of control to facilitate control of the curing depth profile and hence provision of the microstructure pattern. For example, controlling the irradiation intensity and/or duration affects the ultimate curing depth profile and the subsequent microstructure pattern. In the example illustrated in  FIGS. 11B and 11C , the correspondence between the irradiation intensity profile and the curing depth profile is matched or substantially matched. Specifically, the curing depth profile is a sawtooth curing depth profile  1158  corresponding to the sawtooth irradiation intensity profile  1156 . The sawtooth curing depth profile  1158  is achieved by undertaking the step  1106  of removing the uncured photocurable material. In another example, the correspondence may not be matched or substantially matched. For instance, where the photocurable material is irradiated with the sawtooth irradiation intensity profile  1156 , and is continued to be bottom-up cured after the tips of the saw tooth reaching the full height of the photocurable material layer, the resulting curing depth profile may correspond to a trapezoidal profile. 
     Bottom-up curing may be achieved in one of several ways. In one arrangement, the bottom-up curing relies on the presence of oxygen in the atmosphere to facilitate the bottom-up curing. In particular, at least some part of the photocurable material undergoes inhibited curing supressed by oxygen diffused into the photocurable material. The diffused oxygen inhibits polymerisation of photoinitiators in the photocurable material. Under atmospheric conditions, atmospheric oxygen diffuses more into an upper portion (i.e. distal from the exterior surface) of the layer of photocurable material and less into a lower portion (i.e. proximal to the exterior surface) of the layer of photocurable material. In this example, the exterior surface may be that of an aircraft, and the atmospheric oxygen may be provided while the aircraft is held in a hangar. The diffused oxygen and the consequent inhibited curing causes differential curing rates within the layer of the photocurable material. The differential curing rates include a higher curing rate towards the first side and a lower curing rate near the second side. Where the coating is relatively thick, the oxygen inhibition may only be measurable or effective to a threshold depth, below which the photocurable material is allowed to cure with no or little oxygen inhibition. Below the threshold depth, curing becomes more difficult because of attenuation of the light/radiation as it penetrates. This attenuation can be caused by absorption into the polymer itself and/or absorption by pigmentation in the coating. 
     In another arrangement, as a skilled person would appreciate, the exterior surface may be placed in a controlled environment having oxygen pressurised at a predetermined level to control the level of oxygen diffusion and hence controlling the inhibited curing. In yet another arrangement, as a skilled person would appreciate, the exterior surface may be placed in a controlled environment having reduced oxygen level to reduce bottom-up curing or the range over which oxygen penetrates below the coating the surface. 
     Post-Processing 
     The described method  1100  may further include post-processing steps. Subsequent to formation of the microstructure pattern in step  1106 , the method  1100  may include subtractive processing steps or additive processing steps of at least a part of the substrate  1154  where cured photocurable material is absent. As illustrated in  FIG. 16A , the top diagram represents an output of the method  1100  after the step  1106 . The output has a microstructure pattern formed by cured photocurable material  1600  on the top surface of the substrate  1154 . The top surface of the substrate  1154  also includes areas  1602  where the cured photocurable material  1600  is absent. With substrative processing illustrated in  FIG. 16A , the method  1110  further includes removing some of the substrate  1154  by, for example, etching or sand-blasting the top surface of the substrate  1154  and subsequently removing the cured photocurable material  1600 . The output of the subtractive processing is a substrate-only material that includes a microstructure pattern corresponding to the microstructure pattern of the output of step  1106 . Alternatively, with additive processing illustrated in  FIG. 16B , the method  1110  further includes adding additional substrate material by, for example, depositing the additional substrate material on the top surface of the substrate  1154  and subsequently removing the cured photocurable material  1600 . The output of the additive processing is a substrate-only material that includes a microstructure pattern corresponding to (the negative of) the microstructure pattern of the output of step  1106 . 
     The described arrangements of  FIGS. 6-15  overcome at least some of the production difficulties inherent in the arrangements of  FIGS. 2-5 . For example, in one arrangement, the substrate  43  is the top coat of the exterior surface of an aircraft. As another example, the arrangements of the system illustrated in  FIGS. 12 and 14  allow creation of a microstructure pattern without the need for multiple-exposure 
     A characteristic of the roller apparatus, as illustrated in  FIGS. 6-10  and its contactless nature, is that the roller apparatus can be applied to complex curved surfaces and to the windows of aircraft, thereby ensuring both greater coverage and drag reduction. The rollable system  1400  illustrated in  FIG. 14A  as well as the robotic system  1450  illustrated in  FIG. 14C  and described in corresponding paragraphs also provide a similar characteristic. 
     The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention. 
     The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “including” or “having” and not in the exclusive sense of “consisting only of”.