Solid surface free energy analysis using inkjet droplets
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Solid surface free energy analysis using inkjet droplets Kazuhiro Fukada1, Hayato Sakai2, Taku Hasobe2,3, Takashi Masuda4 and Tatsuya Shimoda1,4 1 School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, Japan 2 Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa, Japan 3 PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, Japan 4 Japan Science and Technology Agency, ERATO, Shimoda Nano-Liquid Process Project, 2-5-3 Asahidai, Nomi, Ishikawa, Japan ABSTRACT We measured the surface free energy of a substrate by transmission electron microscopy (TEM) using sub-millimetre-sized inkjet droplets. By employing two types of TEM grids with different surface free energies, we investigated the relationship between the surface energy and the patterns of an organic solution dried on the grids. We confirmed that the generation of the porphyrin hexamer [(H2PAC15)6TPh] patterns was affected by the surface free energy of the TEM grid. INTRODUCTION Contact angle measurement is the conventional method used for calculating surface free energy, which is import for understanding liquid processes including drying phenomena. However, to date, the spatial resolution of contact angle measurement has been limited to several millimetres because of the droplet size. It was recently found that drying a solution of porphyrin on a transmission electron microscopy (TEM) grid results in self-assembled patterns1,2. In these reports, the effects of the molecular structure of the solute on the drying patterns have been investigated. However, these studies did not examine the relationship between the generated pattern and the surface free energy of the TEM grid. This is because it has been difficult to measure the contact angle on a TEM grid, whose size (diameter 3 mm) is smaller than the typical droplet size. In this study, we show that sub-millimetre-sized inkjet droplets can be used for contact angle measurement. Using the proposed method, we measure contact angles, and therefore the surface free energy of TEM grids at ~100 μm resolutions. THEORY Surface free energy was calculated on the basis of the theory presented by van Oss3,4. According to this theory, in the calculation of the surface free energy, two intereactions γLW and γAB are considered (equation (1)).
γ = γ LW + γ AB
…(1)
γ
AB
= 2 γ +γ −
…(2)
LW
AB
γ : Lifshitz–van der Waals interaction γ acid–base interaction + γ : electron-acceptor surface tension parameter γ−: electron-donor surface tension parameter By combining equation (1) with the Young–Dupré equation,
γ L (1 + cos θ ) = WA
…(3)
γL: surface tension of liquid θ: contact angle WA: work of adhesion we obtain the following relationship between the contact angle and the surface free energy.
)
(
γ L (1 + cos θ ) = 2 γ SLW ⋅ γ LLW + γ S+γ L− + γ S−γ L+ ….(4) Here, there are three unknown parameters. To solve these equations, we need to measure contact angles by using
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