Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
pubs.acs.org/jchemeduc
Chemical Curiosity on Campus: An Undergraduate Project on the Structure and Wettability of Natural Surfaces Anthony Katselas,† Alice Motion,† Chiara O’Reilly,‡ and Chiara Neto*,† †
School of Chemistry and Sydney Nano Institute, The University of Sydney, Sydney, New South Wales 2006, Australia Department of Art History, The University of Sydney, Sydney, New South Wales 2006, Australia
‡
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S Supporting Information *
ABSTRACT: An experiment which investigates the wettability of natural surfaces was developed for undergraduate students, which allows them to explore concepts of surface structure, wettability, superhydrophobic surfaces, and self-cleaning. The students choose their own samples of leaves and flowers present in their local environment, examine their surface structure and chemistry by microscopy and contact angle measurements, and, using their understanding of the literature on the wettability of rough surfaces, relate the two. The experiment is student-focused in that it gives students agency in choosing the samples to study, increases their confidence, and develops curiosity in their local environment.
KEYWORDS: Upper-Division Undergraduate, Inquiry-Based/Discovery Learning, Interdisciplinary/Multidisciplinary, Physical Chemistry, Materials Science, Surface Science, Hands-On Learning/Manipulatives
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Characterizing a surface’s wettability is an important aspect of biomimetic research. Wettability provides qualitative information on the chemistry of a material’s surface, since the degree of wetting is dependent on the intermolecular interactions between the surface and a liquid droplet.12 When water droplets only partially wet the surface, the surface energies of the relevant liquid−vapor, liquid−solid, and solid− vapor phases achieve a balance, resulting in a contact angle measured at the three-phase contact line (solid/liquid/ vapor),12 as shown in Figure 1. The equilibrium contact angle θ shown in Figure 1 is defined by Young’s equation:
lant and animal species have evolved over centuries to survive in harsh yet bountiful conditions. Plant species such as the leaf of the sacred lotus have long been known to scientists for their extreme water repellency and self-cleaning properties, which provide a functional basis for survival.1−4 These properties are made possible by the intrinsic hydrophobicity of epicuticular wax crystals which adorn the plant’s leaf surface.5 On their own, waxy hydrocarbon-based coatings provide a chemical basis of moderate water repellency.6 Yet, it is the micro- and nanoscale roughness of these waxy crystal structures that achieves the property of superhydrophobicity.1,7 Superhydrophobicity is one of a number of coveted surface properties found in nature. Antibiofouling ability, antibacterial properties, self-healing ability, structural coloration, and dry adhesion are other well-known properties of interest.8−11 The field of biomimetics focuses on studying biological systems displaying these properties, taking inspiration, and using the properties to solve real world problems.2 Biomimetics is an interdisciplinary field, which brings together engineering, chemistry, and biology, making it an interesting topic for many students. However, biomimetics is seldom taught as part of undergraduate science degree programs. Surface chemistry provides the basic characterization tools used in biomimetic research and offers the benefit of introducing biological system characterization as a final undergraduate year research project. The undergraduate research project described here has inspired students into pursuing further research studies and offers academic staff the opportunity to explore new research directions. © XXXX American Chemical Society and Division of Chemical Education, Inc.
Figure 1. (a) Schematic of a liquid droplet on a solid substrate, with the equilibrium contact angle θ shown, taken at the point where the liquid/vapor contact line meets the solid interface. (b) A droplet suspended in the Cassie−Baxter state, in which the space between surface features contains air pockets. (c) A droplet in a collapsed Wenzel state partially penetrates the surface roughness, replacing the air pockets with water, resulting in droplet pinning. Received: April 3, 2019 Revised: June 3, 2019
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DOI: 10.1021/acs.jchemed.9b00324 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education cos θ =
Laboratory Experiment
γsv − γsl γlv
(1)
where γsv, γsl, and γlv represent the surface energy of the solid− vapor, solid−liquid, and liquid−vapor interfaces, respectively.12 As real surfaces have heterogeneity and roughness, in practice multiple contact angles are measured when the contact line moves over the surface, and this is visualized by measuring contact angle hysteresis, the difference between the advancing and receding contact angle. The advancing contact angle, θA, is measured by increasing the water droplet volume and measuring the maximum contact angle before the contact line moves outward; similarly, the receding contact, θR, is measured by decreasing the droplet volume and measuring the minimum contact angle before the contact line moves inward.12 Atomically smooth hydrophobic surfaces typically display a low hysteresis of around 10−15°,13,14 with more heterogeneous surfaces displaying higher hysteresis values.2,12 The effect of surface roughness is generally to amplify the wetting character of a surface and can be explained (at least for large droplets) by the Wenzel and Cassie−Baxter models, depicted in Figure 1.2,15,16 In the Wenzel wetting state, a water droplet may show a very high contact angle (about 150°), but as the water partially penetrates the roughness of the surface, high droplet pinning and, therefore, higher contact angle hysteresis (>20°) result (Figure 1c).2 In the Cassie−Baxter model the droplet is suspended over a mattress of air pockets trapped within the roughness, and this limits droplet contact to the top of roughness peaks. Functionally, this minimizes droplet pinning, reduces contact angle hysteresis to a few degrees, and aids droplet roll-off at low tilt angles.2 There are only a few papers that have introduced in the chemical education literature the wettability of natural surfaces,6,17 the concept of self-cleaning,18,19 and the relationship between hysteresis and surface heterogeneity.19 Our experiment is novel in that it brings these concepts together across a broad range of natural plant species while also incorporating microscopic inspection of samples. Our experiment mimics more closely than previous works methods used in biomimetic research,17,20−23 and it gives an opportunity for inquiry-led learning, by guiding students to relate the microand nanostructure of natural surfaces to their wetting properties, and to deduce wetting states from their contact angle hysteresis values. Primarily the students work toward identifying whether biological plant species chosen by them are naturally superhydrophobic and, if so, if they also possess self-cleaning properties. To demonstrate this understanding, the concepts are applied in the laboratory using samples, leaves and flowers of plants available in the local area, collected by students and brought back to the laboratory for analysis using available instrumentation. The plant samples were selected on the basis of species available on campus at the University of Sydney or accessible locally (Figure 2). In other countries, local species could be used. Species that appear hairy or rough are most likely to provide interesting microscopic structure that is easily visible under the optical microscope, and these samples are more likely to have interesting wetting properties. Core skills in material science, optical and scanning electron microscopy (where available), are developed through this practical experiment. Further skills in surface characterization using contact angle measurement are practiced, realizing the functional implications of surface roughness on natural
Figure 2. Photographs of the flannel flower (a), jacaranda flower (b), kangaroo paw flower (c), and the banksia cob (d), collected prior to analysis.
surfaces, their superhydrophobicity, and their self-cleaning properties. Students also gain an appreciation of cleanliness when working in the field of surface chemistry. Seven undergraduate students have tested the experiment as individuals and in groups and repeated the contact angle measurements multiple times. The full experiment takes approximately 12 h to complete and was originally developed as a summer research project to engage final year undergraduates with higher research. Following its success, the experiment was offered as an advanced first-year practical module. This experiment is best performed individually over four sessions as outlined in the Supporting Information.
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EXPERIMENTAL OVERVIEW
Sample Preparation for Optical Microscopy and Contact Angle Measurement
Detailed instructions are provided in Supporting Information. The samples studied were the flowers of four species: Anigozanthos f lavidus (kangaroo paw) and Banksia serrata (banksia) flowers were collected from the Sydney University campus, and the Actinotus helianthi (flannel flower) was purchased from Sydney Wildflower Nursery, Heathcote, NSW, Australia. The introduced Jacaranda mimosifolia (jacaranda) was another species chosen for its historical association with Sydney University and was also collected on campus. On collection, surface contamination was minimized by using gloves and tweezers, and samples were then stored in sealed plastic vials. The samples were carefully dissected using tweezers and scissors or a razor blade. The sample size for contact angle measurement was decided on the basis of the sample structure, ensuring the flattest and largest surface area was available (several cm2, if possible). Students must pay attention to reduce contamination of the samples; this includes gloves for handling samples, containers, or tweezers, using clean glassware and ultrapure water. Samples were then fixed to a glass slide with double sided tape and pressed firmly down at the edges to provide adequate adhesion. Samples were characterized immediately after collection to gather results based on the native state of the species. Optical Microscopy
Samples were imaged using an optical microscope in reflection mode (Nikon, Eclipse LV150) and viewed using a 5× objective. Any image demonstrating the microscopic surface structure was captured, and greater magnification objectives were used where possible to magnify surface characteristics at the microscopic level. Contact Angle Goniometry
Contact angles were measured using a KSV Cam 200 goniometer. The static contact angle values were calculated B
DOI: 10.1021/acs.jchemed.9b00324 J. Chem. Educ. XXXX, XXX, XXX−XXX
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by fitting the water droplet baseline in the instrument software utilizing the Young−Laplace model. Droplets of 10 μL (or larger) of ultrapure Milli-Q water were used, containing 0.22 μm3) and having a resistivity of 18.2 MΩ cm. Advancing and receding contact angles were also measured to determine the hysteresis by the mentioned contact angle goniometer. If an equivalent instrument is not available, a cell phone camera is sufficient to capture an image of the water droplets, which is then analyzed using the freeware ImageJ software to measure contact angles as outlined by Lamour et al.24 A tilting table apparatus can also be easily assembled from basic laboratory equipment as per the work of Haines et al., which can replicate the results of advancing and receding contact angle measurements.19 Samples Preparation for Scanning Electron Microscopy
Samples were prepared for scanning electron microscopy (SEM) using two approaches. The first approach was by drying overnight in a vacuum desiccator, which produces good images, but the structures appeared dehydrated. The second method was by a biological specimen fixation protocol which can be used to achieve a more native appearance, but this should be performed under the supervision of a laboratory demonstrator. The dried samples were mounted on SEM stubs using conductive carbon adhesive. Conductive silver paint was used where greater contact was required, and then, the samples were gold coated. Samples were imaged using a JEOL tabletop SEM under high vacuum, aiming to capture the micro- or nanostructures. Where an SEM is not available, a stronger emphasis can be placed on the optical microscopy work. In this case, students focus on the qualitative aspects of microscale structures: at magnifications of ∼100× they may identify features that contribute to roughness, such as trichomes, which promote the superhydrophobic wetting states.
Figure 3. Optical micrographs displaying the native morphology of the flannel flower (a), jacaranda flower (b), kangaroo paw flower (c), and Banksia serrata flower (d). Image insets depict the contact angle measurement on each sample.
trichomes, segmented into three portions. In Figure 3b the attachment point of the trichomes to the flower epicuticle appears dark blue, and the trichomes point downward, becoming more transparent toward the tips. The vertical alignment of the trichomes corresponds to the vertical downward orientation of the flowers on the tree. The features were oriented in the vertical plane (Figure 3b). The spacing measured between surface features was regular, 109 ± 26 μm. For each of the species, the surface was greasy to the touch and, in the case of the banksia, left behind a waxy residue on contact. This qualitative observation suggests that the surface features may be made of a hydrocarbon-based material, which is typical of epicuticular waxes.4,6,20 The second component of the experiment required students to measure contact angles and contact angle hysteresis. Static contact angles were measured on each plant species a total of six times per species, across multiple samples, and an average and standard deviation reported. The results are shown in Table 1.
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HAZARDS During sample collection, students should wear gloves to prevent any allergic irritation during contact with plant species. The handling of glass slides and the use of razor blades should be carried out with extreme caution; disposal should be in a designated sharps receptacle.
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RESULTS AND DISCUSSION The four different flower species, jacaranda, kangaroo paw, flannel flower, and banksia, were readily available at the time of the experiment. The choice of samples allowed a comparative study of the wettability properties of biological samples. After slide preparation, the samples were analyzed using optical microscopy first to observe the native surface morphology as shown in Figure 3. The flannel flower, kangaroo paw, and banksia flowers all displayed similarities in their dense, hairy surface coverage with hairy projections recognized as trichomes, visible on the microand macroscales. The flannel flower displayed a relatively uniform trichome length, with each primary structure branched, and displaying a dense surface coverage (Figure 3a). The kangaroo paw displayed fewer but larger trichome structures than the flannel flower, with numerous branches, leading to much longer and rigid projections with greater variation in trichome length (Figure 3c). The banksia flower displayed waxy trichome hairs which at the surface were heavily kinked. Much longer and straighter hairs were seen to be layered over the top (Figure 3d). The jacaranda displayed
Table 1. Summary of Contact Angle and Contact Angle Hysteresis Measurements with the Associated Standard Deviation Values Surface Flannel flower Jacaranda flower Kangaroo paw flower Banksia flower
Contact Angle/deg
Hysteresis/deg
± ± ± ±
25 ± 2 41 ± 5 n/a 22 ± 6
154 157 137 150
1 6 13 8
Considering the nature of epicuticular waxes, all samples were not surprisingly hydrophobic with most samples displaying contact angle values in the superhydrophobic regime (θ > 150°). The kangaroo paw flower was the sample with the lowest static contact angle value, and the value was affected by large measurement variability, due to the irregular surface structure. Overall, the scatter in the measurement of static contact angle was high (except in the flannel flower) due the irregular surface features. Both the flannel flower and C
DOI: 10.1021/acs.jchemed.9b00324 J. Chem. Educ. XXXX, XXX, XXX−XXX
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sample fixation for SEM analysis and indirectly suggests that the surface projections were heavily hydrated in their native state. If students do not have access to biological sample fixation, conclusions may still be drawn from vacuum-dried samples, as clear nanoscale surface features can be observed, and inferences can be made from the correlation with optical micrographs. SEM micrographs of the remaining samples prepared through vacuum drying are shown in Figure 5. All of
banksia displayed comparable and high contact angle hysteresis values, which suggest a rough surface chemistry and/or an irregular surface. The high value of contact angle hysteresis points to a Wenzel state,2,15,16 whereby the droplets make direct contact with the full length of the surface features, penetrating the roughness. This type of topography is unlikely to trap air pockets between the features, and does not produce an effective self-cleaning effect. The jacaranda flower displayed the fewest number of surface projections and overall had flatter topography, and it had the highest value of hysteresis of all samples. Contact angle hysteresis could not be measured on the kangaroo paw, due to the spontaneous collapse and spreading of the water droplet within the porous flower structure when increasing droplet volume. For the SEM analysis component of the experiment, the jacaranda flower was prepared using both the biological fixation and vacuum drying methods for comparison (Figure 4). The fixed samples clearly displayed finger-like trichome
Figure 5. SEM micrographs of the banksia serrata flower (a, b), flannel flower (c, d), and kangaroo paw (e, f).
the samples display hair-like or branch-like projections. The banksia flower (Figure 5a) displayed trichomes which were more hair-like, whereas the flannel flower (Figure 5c) trichomes appeared to branch from a central point into longer branches, and the kangaroo paw (Figure 5e) appeared to branch off from a central projection into numerous short branches. At a higher magnification, the banksia flower (Figure 5b) and flannel flower (Figure 5d) trichomes displayed a nanoscale wrinkled texture, unlike the kangaroo paw, which appears smooth on the same scale (Figure 5f). It is not possible to conclude that the nanoscale wrinkles observed are native features and not a result of the vacuum drying process. What is clear, however, is that the structures are much more robust than those of the jacaranda flower, retaining their native morphology at the microscale. It can be inferred that the smoother surface of the kangaroo paw led to increased surface wetting, and indeed, the kangaroo paw had the lowest water contact angles out of all the species (Table 1). In the lotus leaf, it is particularly the hierarchical (micro- and nanoscale superimposed) roughness that is effective at trapping air, achieving the Cassie−Baxter state. The self-cleaning ability is related to the Cassie−Baxter state, because only in this configuration does the topography allow capillary forces to direct any dirt resting on the structure to adhere to the water droplet.4 Self-cleaning species typically have an ultralow hysteresis measurement of
