Laboratory Experiment pubs.acs.org/jchemeduc
A Novel General Chemistry Laboratory: Creation of Biomimetic Superhydrophobic Surfaces through Replica Molding Samuel Verbanic, Owen Brady, Ahmed Sanda, Carolina Gustafson, and Zachary J. Donhauser* Department of Chemistry, Vassar College, Poughkeepsie, New York 12601, United States S Supporting Information *
ABSTRACT: Biomimetic replicas of superhydrophobic lotus and taro leaf surfaces can be made using polydimethylsiloxane. These replicas faithfully reproduce the microstructures of the leaves’ surface and can be analyzed using contact angle goniometry, self-cleaning experiments, and optical microscopy. These simple and adaptable experiments were used to develop a novel general chemistry laboratory exercise, designed to introduce students to the current fields of materials science and biomimetics, and reinforce concepts such as intermolecular forces and surface tension. KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Physical Chemistry, Hands On Learning/Manipulatives, Materials Science, Physical Properties, Surface Science
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chemistry lab sequence, as it provides students with a hands-on introduction to materials science and biomimetics, yet it retains strong connections to general chemistry topics such as intermolecular forces, surface tension, and polymers. In this exercise, the structure and properties of superhydrophobic leaves are reproduced using replica molding with polydimethylsiloxane (PDMS), an inexpensive and easy to use polymer that has been widely applied in many microfabrication techniques such as injection molding and microcontact printing. After fabrication of the biomimetic surfaces, the replicas can be investigated using a suite of techniques to investigate their wetting, self-cleaning, and morphological properties. Furthermore, with a more thorough treatment of surface tension and wetting, this experiment could be readily adapted for an upper-division physical chemistry lab.
atural biological systems have long been vital sources of inspiration for scientists and societies alike. Biomimicry is evident in many widely used products, for example, VELCRO was designed after a Swiss engineer noticed how burrs clung to his dog’s fur,1 and engineers are currently designing dry adhesives modeled after gecko feet, which exhibit powerful yet easily reversible adhesion forces.2 Pioneering biomimetic techniques are producing promising results in current fields such as nanomedicine and materials science.3,4 One class of biological systems that has been the inspiration for biomimetic materials is the superhydrophobic surfaces of some plant leaves. Superhydrophobicity, also referred to as the lotus ef fect, refers to surfaces with water contact angles of over 150 degrees,5 with the result that water readily beads up and rolls off of these surfaces without wetting. The effect is produced by an organized, hierarchical surface structure, with microscale spires covering the surface of the leaf, and those spires covered in smaller nanoscale spires.6 Chemical hydrophobicity alone can lead to water contact angles of up to ∼120°, whereas a hierarchical surface structure is required to achieve the high contact angles characteristic of superhydrophobic surfaces.7 The topography allows water droplets to sit on the tips of the spires, leaving a pocket of air between the water and base of the leaf. The result is minimal contact between water and the surface, increased contact angles, lowered adhesion forces, and water drops that roll off at very shallow angles. Additionally, dust and other particulates on the surface can be picked up by the rolling droplets, making the surfaces naturally self-cleaning.5 This paper describes biomimetic replication of superhydrophobic taro (Colocasia esculenta) and lotus (Nelumbo spp.) leaf surfaces,8 adapted as a laboratory experiment for undergraduates in first-year general chemistry courses. This experiment is an interesting departure from a typical general © 2014 American Chemical Society and Division of Chemical Education, Inc.
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EXPERIMENTAL OVERVIEW In our first implementation of this experiment, it was run in a class of 20 students, with students working in pairs. The experiments described here require at least one partial lab period for casting of the primary (negative) polymer molds, and a full lab period to create secondary (positive) polymer molds and analyze them. Negative mold casting requires minimal hands-on lab time and can easily be done concurrently during another lab exercise. The PDMS negative molds cure for a week at room temperature until the second lab period, which requires about 4 h for curing of PDMS positive molds and analysis of the polymer surfaces. The basic replication scheme is depicted in Figure 1, with a detailed procedure in the Supporting Information. After the biomimetic replicas are made, a number of simple techniques can be used to characterize the artificial polymer leaf Published: July 17, 2014 1477
dx.doi.org/10.1021/ed4007056 | J. Chem. Educ. 2014, 91, 1477−1480
Journal of Chemical Education
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Laboratory Experiment
RESULTS AND DISCUSSION The analytical components of this experiment are designed to observe, test, and better understand the superhydrophobic effect created through biomimetic replication, to provide students with exposure to new techniques, and to engage students in the development of a novel experimental method. Contact angle goniometry is a simple but powerful tool, which quantifies hydrophobicity and provides striking qualitative visual differences between superhydrophic and hydrophobic surfaces. The self-cleaning exercise gives students the opportunity to develop their own experiment as they build their apparatus and devise a self-cleaning quantification scheme. Optical microscopy (or SEM if available) allows direct observations of the powerful adaptive characteristics of taro and lotus leaf surfaces, and the fidelity of their replicas. Contact angles of plain flat PDMS and biomimetic molds were compared using a commercial contact angle goniometer, but inexpensive home-built instruments may also be constructed during the lab period.11,12 The images in Figure 2 and Figure 1. Schematic of fabrication procedure of PDMS Lotus replicas.
surfaces. We note that each characterization technique represents an independent experiment, such that they can be included or omitted depending on the availability of time and equipment. We have tested the following techniques, each described in more detail below, and outlined in the Supporting Information: (1) contact angle measurements to measure hydrophobicity; (2) self-cleaning measurements using a simple homemade apparatus;9 (3) optical microscopy to visualize the microtextured surfaces. If available, scanning electron microscopy (SEM) can also be used to visualize PDMS surfaces. Two types of superhydrophobic leaves were tested for this experiment, and the results below demonstrate that lotus leaves are ideal for these experiments, but taro leaves work nearly as well. Lotus molds were found to be more superhydrophobic, durable, and consistent, but these plants are seasonal and difficult to obtain in some areas. Taro plants are inexpensive and widely available year-round, and while their replicas are very hydrophobic, we found that some of the superhydrophobicity was lost in replication. All of the experiments described below have been tested with both types of leaves, and both are suitable to achieve the desired results. Additionally, although we only tested two types of leaves, there may be other superhydrophobic leaves and surfaces with microtextures that can replicated using PDMS.10 Students were evaluated for this experiment based on their written responses to prelab and postlab questions, which are provided in the Supporting Information.
Figure 2. (Left) Side view of a water droplet on plain PDMS surface. (Middle) Water droplet on PDMS taro replica. (Right) Water droplet on PDMS lotus replica.
Figure 3. Contact angles of various surfaces with standard deviations. Numbers are averages of three measurements.
the data in Figure 3 reveal that lotus and taro leaf PDMS replicas had significantly larger contact angles than plain PDMS. Biomimetic copies of taro and lotus leaves had comparable contact angles to the natural leaves, indicating successful replication of leaf structure and superhydrophobic character. The taro leaf positive molds yielded hydrophobic contact angles (139° on average); however, this contact angle was ∼9% lower than that of the actual taro leaf. In contrast, lotus leaf positive molds had approximately the same contact angle as their natural counterparts (>150°). The experiment showed that both taro and lotus molds were significantly more hydrophobic than plain PDMS molds (33% increase in lotus, and 22% increase for taro). Although the superhydrophobic surfaces were not analyzed chemically beyond contact angle measurements, the high contact angles observed are characteristic of a hierarchical surface structure on the naturally hydrophobic PDMS surface. In addition, the negative molds had contact angles comparable to that of flat PDMS (
