Citation for published version (APA): Wal, B. P. V. D. (2006). Static and dynamic wetting of porous Teflon surfaces s.n.

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1 University of Groningen Static and dynamic wetting of porous Teflon surfaces Wal, Bouwe Pieter van der IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2006 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Wal, B. P. V. D. (2006). Static and dynamic wetting of porous Teflon surfaces s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Chapter 4 Spin-Coating of Teflon R AF Solutions in a Vapour Saturated Environment This chapter describes a technique involving breath figures to structure Teflon R AF films. 4.1 Introduction Breath figures form when a vapour condenses onto a cold surface, for example eye-glasses become fogged when a person enters a warm room 31, 32 from the cold. The condensed vapour can either form an uniform film, or an assembly of drops, depending on the wetting behaviour of the surface. 33 When the solvent of a polymer solution rapidly evaporates in moist air, either by using a low boiling point solvent or by using the spin-coating technique, water condenses onto the surface of the solution because of evaporative cooling and forms an array of drops. After all the solvent and water have evaporated, the obtained film surface features holes that are a signature of the intermediate water drops. For some polymers, such as polystyrene dissolved in methanol, a hexagonally 34, 35 ordered structure (or honeycomb structure) of holes can be obtained.

3 32 Spin-coating of Teflon R AF in a Vapour Saturated Environment This chapter describes a method to increase the roughness of a Teflon R AF film using breath figures. Teflon R AF is a copolymer of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) and a fluorine-containing monomer. In this case Teflon R AF 1600 was used, which is a copolymer of PDD and tetrafluoroethylene. Teflon R AF was preferred over Teflon R because Teflon R AF can be dissolved in a range of perfluorinated solvents. As a solvent, FC-75 (Perfluoro-(2-perfluoro-n-butyl)tetrahydrofuran) supplied by 3M was used. A smooth Teflon R AF film has a water contact angle of 104. For this work, the spin-coating technique was used to prepare the films. A drop of the polymer solution was placed onto a substrate, which is rotated at a fixed speed. Normally great care is taken to produce extremely smooth films. One problem that can arise is the condensation of water onto the polymer solution during spinning. That is why the spin-coater chamber is typically flushed with dry nitrogen gas. In this case however, we made use of, and even tried to enhance, this phenomenon. 4.2 Experimental Set-up For this technique, a spin-coater (Headway Research Inc. i, model PWM32) was used. To enhance the condensation of the vapour on the Teflon R AF solution, the standard Teflon R rotating chuck was replaced with one made from copper, which was cooled in a fridge to about 5 C before each experiment. A 10 ml beaker filled with a liquid was placed into the spin-coater chamber, next to the rotating chuck with a silicon substrate. The beaker was heated by a peltier-element in order to form a vapour in the chamber. By keeping the liquid temperature constant, the vapour saturation in the spin-coater chamber covered with a lid was kept constant. A drop of a 3 % Teflon R AF solution was placed onto the silicon i Headway Research Inc., Garland, USA

4 Chapter 4 33 substrate. After 30 seconds, the substrate was rotated for about 30 seconds. Subsequently, the lid was removed to allow the vapour to escape. A schematic drawing of this process is shown in Figure 4.1. The ex- Figure 4.1: Experimental setup: 1) A boiling liquid is placed into the spin-coater chamber next to the cooled chuck with a Teflon R AF solution covered substrate on it. 2) Drops of liquid condense on top of the cold Teflon R AF solution during spin-coating. 3) A Teflon R AF film with holes remains after this procedure.

5 34 Spin-coating of Teflon R AF in a Vapour Saturated Environment periments were performed with a number of different vapours. The first experiments were performed using water, later experiments used ethanol, toluene, chloroform and fluorotrichloromethane vapours. The chemicals used in this study are shown in Table 4.1. Chemical Manufacturer Quality Boiling Point ( C) Teflon R AF 1600 DuPont FC-75 3M 102 Ethanol Aldrich 95 % 78 Chloroform Aldrich >99 % 62 Toluene Sigma 99.5 % 110 Fluorotrichloromethane Fluka 99.5 % 24 Table 4.1: List of chemicals used. 4.3 Results For water vapour, the effect of the relative humidity on the surface morphology was investigated. The spin-coating speed was kept constant at 10,000 rpm, and the water vapour pressure in the spin-coater chamber was varied by regulating the temperature of the water bath. AFM scans of the surfaces show the surface texture changes with the relative humidity. The surface spin-coated in a vapour atmosphere of 87 % relative humidity is relatively smooth compared to the surfaces spin-coated in an atmosphere of 92 % or 97 % relative humidity (see Figure 4.2). The change in surface texture has a significant effect on the contact angle of a drop of water on that surface. The change in the advancing contact angle as a function of the relative humidity is shown in Figure 4.3. A clear jump in the contact angle can be seen at a relative humidity of

6 Chapter 4 35 Figure 4.2: The effect of the relative humidity on a Teflon R AF structure. about 90 %. In a second series of experiments, the effect of the spin-coating speed on the surface texture of a spin-coated Teflon R AF film and the associated contact angle of water on this surface was investigated. This series of experiments was performed using water, ethanol, toluene, chloroform and fluorotrichloromethane vapours. The liquids were heated to the boiling temperature of the liquid, to ensure the vapour saturation of the spincoater chamber. The spin-coating speed was varied from 1,000 rpm to 10,000 rpm - the maximum rotation speed that can be achieved with the spin-coater used in our experiments. AFM scans of the films spin-coated at 1,000 rpm in a water vapour saturated environment show several holes with a diameter of circa 5 µm, surrounded by holes with diameters of 1 µm or less (see Figure 4.4). For rotation speeds of 5,000 rpm and 10,000 rpm only holes with diameters of 1 µm or less were observed. The holes have a depth of approximately 1 µm. The results for the advancing contact angle are shown in Figure 4.5. There is a marked and almost linear increase in the contact angle for both water and chloroform vapour. The increase is the most obvious for chloroform vapour. In the case of ethanol, the contact angle seems not to be influenced by the spin-coating speed. For toluene, a small decrease in the contact angle is seen up to a spin-coating speed of 5,000 rpm.

7 36 Spin-coating of Teflon R AF in a Vapour Saturated Environment C o n ta c t A n g le R e la tiv e H u m id ity Figure 4.3: The effect of the relative humidity on the contact angle of a drop of water on a Teflon R AF surface. For higher spin-coating speeds, the contact angle increases to reach its highest value for 10,000 rpm. The highest contact angles are reached with water and chloroform vapour at a spin-coating speed of 10,000 rpm. The receding contact angle follows a similar trend as the advancing contact angle. The contact angle hysteresis had a typical value of approximately 15. Fluorotrichloromethane vapours were not useful in these experiments because they destroyed the Teflon R AF film during the spin-coating process.

8 Chapter 4 37 Figure 4.4: The effect of the spin-coating speed on a Teflon R AF structure for water vapour. 4.4 Conclusions It is possible to increase the water contact angle of a Teflon R AF film by spin-coating in a vapour saturated environment. The obtained film surface features holes that are a signature of the intermediate condensate drops. For films spin-coated in a humid environment, the surface texture changes with the relative humidity. A film spin-coated in a vapour atmosphere of 87 % is relatively smooth compared to films spin-coated in a vapour atmosphere of 92 % and 97 % relative humidity. The surface texture also changes as a function of the spin-coating speed. In a water vapour saturated environment, for a film spin-coated at 1,000 rpm several 5 µm holes surrounded by holes with 1 µm diameter or less are observed. For films spin-coated at 5,000 and 10,000 rpm, only holes with diameters of 1 µm or less were observed. The highest contact angles were achieved by using water or chloroform vapours at a spincoating speed of 10,000 rpm. Even though the contact angle increased by about 30 compared to a smooth Teflon R AF film, which has a water contact angle of 104, this procedure does not produce surfaces with a sufficient corrugation to achieve a self-cleaning surface.

9 38 Spin-coating of Teflon R AF in a Vapour Saturated Environment C o n ta c t A n g le W a te r E th a n o l C h lo ro fo rm T o lu e n e S p in -C o a tin g S p e e d (rp m ) Figure 4.5: The effect of the spin-coating speed on the contact angle.