Natural structures design

Tạp chí Khoa học - Công nghệ Thủy sản Số 1/2014 TRƯỜNG ĐẠI HỌC NHA TRANG • 39 NATURAL STRUCTURES DESIGN PHÁC HỌA CẤU TRÚC TỰ NHIÊN Kroisova Dora1, Ron Jiri2 Ngày n hận bài: 21/8/2013; Ngày phản biện thông qua: 12/02/2014; Ngày duyệt đăng: 10/3/2014 TÓM TẮT Mục đích của bài báo là nghiên cứu một số bề mặt cấu trúc tự nhiên, thiế t kế thông minh và sá ng tạ o trên bề bặ t củ a chú ng nhằ m ứ ng dụ ng chú ng và o công nghiệp. Bề mặt lá sen (Nelumbo nucifera) được biết đến như một mặt phẳng không dính nước, bề mặt của rêu (Bryophyta) được xem như một bề mặt ưa nước và da cá mập (Carcharodon Carcharias) như bề mặt với các thông số rất tốt về thủy cơ đã được lựa chọn để nghiên cứu và phát họa cấu trúc. Ban đầu tất cả các mẫu được sấy khô trong không khí và sau đó phun lên bề mặt mẫu một lớp hợp kim Au-Pd. Các nghiên cứu về cấu trúc đều được thực hiện trên kính hiển vi điện tử quét VEGA\\TESCAN với độ phóng đại trong khoảng từ 200 đến 100 000 với điện áp gia tốc là 10 kV. Thông qua kính hiển vi điện tử quét để có thêm thông tin về cấu trúc của chúng. Phần mềm ImageTool phiên bản 3.0 của Trường Đại học Texas Health Science Center ở San Antonio đã được sử dụng để phân tích hình ảnh của bề mặt tự nhiên. Công ty KOH-I-NOR PONAS tại Cộng hòa Séc đã hợp tác chọn phác họa các bề mặt cấu trúc này và bây giờ nhiệm vụ của họ là thử nghiệm trong điều kiện thực tế. Từ khóa: cấu trúc tự nhiên và cấu trúc nano, thiết kế, kỹ thuật sinh học ABSTRACT The aim of this work is studying of natural surface structures, their design preparation and subsequent creation of the specifi c surfaces of molds for industrial applications. Lotus leaf surface (Nelumbo nucifera) as a superhydrophobic plant surface, a leaf surface of a common moss (Bryophyta) as a hydrophilic surface and a shark skin (Carcharodon carcharias) as a surface with good hydromechanical parameters were selected in order to show how to study and design the structures. At fi rst all the samples were dried in the air and after that were sputtered by a layer of Au-Pd alloy. The studies of the natural object structures were performed on a scanning electron microscope VEGA\\TESCAN at magnifi cations in the range of 200 to 100000x at 10 kV accelerating voltage. Through the scanning electron microscopy it is possible to get more information about the structures in order to create their design. Software ImageTool version 3.0, The University of Texas Health Science Center at San Antonio was used for an image analysis of the selected natural surfaces. The designed structures of chosen surfaces were prepared in cooperation with KOH-I-NOR PONAS, the Czech Republic Company, and now their functions are tested in real conditions. Keywords: natural structures and nanostructures, design, bionics 1 Assoc. Prof. Kroisova: Centre for Nanomaterials, Advanced Technologies and Innovations, Technical University of Liberec, Czech Republic, Email: dora.kroisova@tul.cz 2 Msc. Ron Jiri: Faculty of Mechanical Engineering, Technical University of Liberec, Czech Republic THOÂNG BAÙO KHOA HOÏC I. INTRODUCTION An increasing amount of researchers have been working on a study of natural plant and animal surfaces from point of view of material compositions and structures as well as processes by which are these objects created. An interest in this type of materials comes out from the fact that life on the Earth has developed for more than 3.5 billion years which has resulted in practically ideal solutions. The natural surfaces usually show multilevel structures beginning on a molecular level through a nano-scale level and ending on a micro-scale level where many different material combinations are in mutual coexistence. Tạp chí Khoa học - Công nghệ Thủy sản Số 1/2014 40 • TRƯỜNG ĐẠI HỌC NHA TRANG Suitable examples are water repellent plant leaves surfaces created by various structures where the microstructure is formed of single surface cells and the nanostructure is formed by wax particles secreted on the cells surface. In some cases there may occurred obvious macrostructure which can even be visible by the naked eye. These structures facilitate to leaves to stay clean because adhered dust and other impurities hinder a process of photosynthesis [1], [2]. The fi rst studied object was a superhydrophobic surface of a leaf of an Indian lotus (Nelumbo nucifera). Other surfaces inspiring to a muse may be for instance a hydrophilic moss surface ensuring water and nutrients absorption without necessity of using a root system. The structures lessening water fl ow resistance could be animal skins - shark skin in case of this study [3]. II. EXPERIMENTS For a microscopic evaluation and following image analysis were chosen samples of the Indian lotus, the moss and the shark skin. All the samples were dried in the air and sputtered by a thin layer of Au-Pd alloy. Observations of the structures were performed on a scanning electron microscope (SEM) VEGA\\TESCAN at magnifi cations in the range of 200 to 100000x at 10 kV accelerating voltage. SEM images of the sample surface structures were used for the image analysis which aim was fi nding dimensions and geometry of the micro and nanoparticles. Software ImageTool version 3.0, The University of Texas Health Science Center at San Antonio was used for the image analysis. The images were tresholded and transferred to binary images, i.e. bright areas (peaks of the surface cells of the lotus) on the SEM images were transferred to white colour and areas among the cells to black colour. The images were purifi ed from noise (spots and smears). Some cells on the SEM images were joined together which would be evaluated as a one cell instead of two ones. This problem was treated by a “watershed” function. After a segmentation of the epidermal cells a cell density ρcells was evaluated. Another step was a contact area ratio between liquid and solid phase fLSmicro evaluation (i.e. a sum of the white areas divided by the whole image area). The same policy was applied to get fLSnano values. A fi gure 1 shows the origin area from which was measured the cells density ρcells and outlines of the contact areas [4]. Figure 1. Upper side of the Indian lotus leaf with the applied image analysis on the SEM image [4] The image analysis evaluated these values: Cells density: ρ cells = 3205 [mm-2] Contact area ratio f LSmicro = 0,101 [-] f LSnano = 0,241 [-] There can be counted which area is equal to a single cell from the cell density ρcells assuming hexagonal layout of the cells. The single cell area multiplied by fLSmicro corresponds with an area which is in contact with liquid from which was derived a contact diameter dcont written bellow: On the basis of the image analysis a model of a form for fabrication has been designed. The value of dcont from the analysis (Fig. 2 up) is adequate to a value of a formations which should be fabricated (Fig. 2 down). A density of the fabricated formations, their pitches and geometric layout should also be ade- quate to the studied natural surface [4]. Figure 2. Models of natural surface structure (up) and its transformation to technical form (down) [4] Tạp chí Khoa học - Công nghệ Thủy sản Số 1/2014 TRƯỜNG ĐẠI HỌC NHA TRANG • 41 III. RESULTS AND DISCUSSION With the usage of the SEM and image analysis, the evaluation of three different types of natural objects was performed. The lotus leaf is known for its specifi c characteristic which is a high hydrophobicity. This characteristic is achieved by the microstructure and the nanostructure of the leaf surface which may also be increased by chemical composition of the surface waxes on the nanoscale level. By the surface analysis, data about the shapes, layouts, dimensions and densities of the cells and the waxes have been achieved. There is no possibility of identical structural analogy fabrication by current conventional methods. A following tab. 1 shows a contact angles and their hysteresis, the epidermal cell densities, the micro and nano contact area ratios and derived contact diameters with heights and pitches between asperities (epidermal cells) for 4 chosen hydrophobic surfaces. Tab. 1. Geometric parameters of chosen hydrophobic plants Plant name Contact angle [°] Contact angle hysteresis [°] Epidermal cell density ρcells [mm-2] Micro contact area ratio fLSmicro [-] Nano contact area ratio fLSnano [-] Contact diameter dcont [µm] Epidermal cell height H [µm] Epidermal cell pitch P [µm] Indian lotus* (Nelumbo Nucifera) 146,7 ± 3,5 1,4 ± 0,9 3205 0,101 0,241 6,6 12,2 17,7 Cock’s-foot* (Dactylis glomerata) 146,7 ± 1,9 2,1 ± 1,3 3764 0,201 0,190 8,3 9,0 16,3 St John’s wort* (Hypericum perforatum) 151,6 ± 1,5 1,7 ± 1,0 1562 0,114 0,249 9,6 11,3 25,3 Poinsettia* (Euphorbia Pulcherrima) 146,3 ± 5,0 1,4 ± 0,9 2022 0,098 0,168 7,9 6,8 22,2 * All the values were measured on the upper side of dehydrated leaves 1. Indian lotus (Nelumbo nucifera) Figure 3. Typical structure of the Indian lotus surface. Convex epidermal cells (the upper image) covered by the tiny wax rods (the lower image). SEM images [4] The Indian lotus is a thermo-philic tropic swamp plant which originates from India. Almost circular leaves emerge from swamp water as well as their blossoms. Despite growing in backwater the lotus remains perfectly clean a dried which is ensured by its hierarchic structure formed by the microscale concave cells with hollow nanoscale wax rods on the cells surfaces. The length of the wax rods is about 600 nm and its diameters (inner and outer) about 100 nm (fi g. 4). Besides being clean, this structure also ensures self-cleaning ability. There was found out by the image analysis that the cell density ρcells is 3205 mm -2 and hollow rods density ρwaxes was approximately 6 mil. mm -2. This nanostructure with such a huge density and a nanoparticles shape is unfabricatable by current technologies. Fig. 4 shows the measured dimensions on models [4]. Tạp chí Khoa học - Công nghệ Thủy sản Số 1/2014 42 • TRƯỜNG ĐẠI HỌC NHA TRANG Although the contact angles and hysteresis of four different hydrophobic leaves are very similar, geometrical parameters of the leaves vary distinctly. This variety may be caused by morphology and geometry of epidermal waxes and their chemical compositions. Wax morphology varies from tubes to platelets with different types of arrangement. Indian lotus possesses hollow tubes with irregular arrangement while St John’s wort has regular platelets with spacing which resembles stars with fi ve tips. St John´s wort microstructure seems to be the most profi table because it has the highest contact angle (151,6 ± 1,5) with the largest dimensions of asperities which is less diffi cult for fabrication and also cheaper. Due to the fi neness, variety and wide morphology of waxes which is currently inimitable it would be useful to choose St John´s wort microstructure dimensions as a default microstructure and try to deposit nanoasperities (PECVD) with varying dimensions and fi nd out the best combination of micro and nanostructure to get the highest contact angle value experimentally. 2. Moss (Bryophyta) Mosses are green nonvascular plants of a small growth with a distinct ability to retain water. The mosses accept water by the whole surface of thallus and distribute it by their water conducting tissues or easily by wettable surfaces. The way how the mosses manage water enables them to use even very little amount of rainfall. Figure 5. SEM image of the moss with the obvious structured surface The image analysis showed an elongated hexagonal alignment of the structure. Dimensions gained from the real structure were used for a fabrication of a form for plastic material injection. The form has been fabricated in many dimensional variations with maintenance of a dimensions ratio and its function has been currently tested in real conditions. Figure 4. Models of the surface structure covering leaves of the Indian lotus - the documentation of the structure [4] Figure 6. Model of the moss surface for the form fabrication Tạp chí Khoa học - Công nghệ Thủy sản Số 1/2014 TRƯỜNG ĐẠI HỌC NHA TRANG • 43 Figure 7. SEM image of the form surface for the moss structure fabrication. Fabricated by KOH-I-NOOR PONAS s.r.o The mosses are interesting for their ability of surface water absorption. The model created according to the SEM images of original moss structure was served for the form surface fabrication. The form has been fabricated in cooperation with Department of Engineering Technology TUL and KOH-I-NOOR PONAS s.r.o. The function of the form has been currently tested in real conditions. 3. White shark (Carcharodon carcharias) A specifi c shark skin structure is covered by fi rm tooth-like scales lessening water fl ow resistance. Figure 8 shows the SEM image of shark skin scales and there are scales dimensions in fi gure 9. A scale width varies from 150 to 200 µm, a distance between longitudal fl utes is about 40 µm and a height of the fl utes varies from 10 to 20 µm. The shark skin surface was characterised on the basis of the image analysis. A new structure (fi gure 10) was fabricated according to the model but regarding to the large difference from its original the new structure hasn´t been tested. Figure 9. Model of the shark skin scale as a basis for the form fabrication Figure 8. SEM image of the shark skin surface with the obvious scale structure [3] Figure 10. SEM image of the new form surface fabricated on the basis of the derived dimensions. Fabricated by KOH-I-NOOR PONAS s.r.o The shark skin was used for modelling of the surface with low resistance against the fl ow in the water environment. The model with the dimensions achieved from the SEM images was used to design of the surface of the fabricated form. From the image above it is obvious that it was not possible to fabricate the form surface on a required level regarding the shape, structure and dimensions. IV. CONCLUSION The described image analysis applied to the images of the natural objects achieved from the scanning electron microscopy turned out to be a suitable method to determination of the surface characteristic parameters such as the size, the shape and the layout of the cells and the waxes. The parameters achieved by the image analysis are suffi cient for models design of the form surfaces fabrication. Tạp chí Khoa học - Công nghệ Thủy sản Số 1/2014 44 • TRƯỜNG ĐẠI HỌC NHA TRANG The form fabrication by today´s conventional technologies can reach only a microscale level shapes but not smaller. Experimental data showed that the most profi table geometry of microscale, based on the chosen leaves is derived from St John´s wort leave (upper side) with the highest angle (151,6 ± 1,5). Diameter of the asperities of such form is 9,6 µm, their height 11,3 µm and pitch 25,3 µm. ACKNOWLEDGEMENTS The research was supported in part by the Project OP VaVpI Centre for Nanomaterials, Advanced Technologies and Innovation CZ.1.05/2.1.00/01.0005 and student´s project SGS TUL. The acknowledgement for cooperation belongs to Ing. P. Kejzlar, prof. P. Lenfeld and workers of KOH-I-NOOR PONAS. REFERENCES 1. D. Kroisova, 2012. Microstructures and Nanostructurees in Nature. In: Progress in Optics, 57: 93 – 132. 2. J. Ron, 2012. The study of surface structures of chosen natural object and potentials of creating their analogies. In: Diploma thesis. 3. K. Koch and W. Barthlott, 2009. Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials. In: Philosophical Transactions of the Royal Society A, 367: 1487- 1509. 4. K. Koch and H.J. Ensikat, 2008. The hydrophobic coatings of plant surfaces: Epicuticular wax crystals and their morphologies, crystalinity and molecular self-assembly. In: Micron, 39: 759-772.