Page 66 - Handbook of Adhesion Promoters

4.2 Surface treatment 59

Figure 4.6. SEM images of different materials after ultrasonic nanostructuring with points of correlation between
surface properties and response of the surface to ultrasonic treatment. Surface hydrophilicity, stability to Red/Ox
reactions, adhesion of surface layers to substrate, stiffness and melting temperature are important to predict ultra-
sonic effect on surfaces. [Adapted, by permission, from Skorb, EV; Möhwald, H, Ultrasonic Sonochem., 29, 589-
603, 2016.]
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ther cleaning with acid created even greater roughness. The elemental analysis showed
that some oxide was removed by alkaline etching but its substantial removal occurred only
after acid cleaning, although some oxygen still remained on the surface, most likely due to
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formation of Al(OH) during alkaline etching. Resultant roughness after degreasing,
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alkaline etching, and alkaline etching followed by acid cleaning was 128, 375, 429 nm,
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respectively. Figure 4.5 summarizes surface changes. Adhesion strength was 4.4, 4.6,
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and 5.6 after degreasing, alkali etching, and full treatment, respectively. The increase in
surface free energy and surface roughness were the reasons for substantial adhesion
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improvement on the samples prepared by alkaline etching followed by acid cleaning.
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The surface cleaning by acoustic cavitation uses strong ultrasonic fields. The ultra-
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sonic cleaning can be used for both cleaning and nanostructuring. The gas clusters may
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expand to form bubble nuclei in the zone of the sound negative pressure. These do not
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completely collapse during the high-pressure phase. They are further expanded in the
next low-pressure phases. This process continues until bubbles reach a maximum critical
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diameter which depends on the ultrasound frequency and solvent. The internal bubbles
upon collapse create transiently temperatures above 5000 K and pressures above 1000 atm
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with cooling rates above 10 K/s. The process of high temperature and pressure can be
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performed at room temperature. When the bubbles collapse near a solid surface, they
form microjets having an estimated speed of hundreds of m/s which may lead to pitting
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and erosion of the surface. Figure 4.6 shows results of nanostructuring of some materials.
Some fundamental aspects of the cavitation processes are still unknown (see question
marks in Figure 4.6). Figure 4.7 illustrates some effects of bubble collapse on a material
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surface.