5:15 PM - 7:15 PM
[O08-P12] Bubble Adhesion on Shark Skin
★Invited Papers
Keywords:Shark skin, Hydrophobicity, Contact angle
The surface of shark skin is covered with numerous microscopic structures that resemble the shape of teeth. These structures consist of grooves that run parallel to the direction of water flow, resulting in a structured roughness. The shape, size, and spacing of these structures vary depending on their location on the body surface and the species. For example, in Carcharhinus brachyurus, the size of the microscopic structure ranges from approximately 0.2 to 0.5 mm, and the spacing is about 20 to 30 µm. Unlike Osteichthyes, sharks belong to the class Chondrichthyes, and the microscopic structures on their surface skin are composed of hard enamel. This structure might enable sharks to reduce water resistance during their swims [1].
In general, when surfaces with fine rough structures contact a water droplet in atmospheric conditions, tiny air bubbles can be retained within the grooves, exhibiting hydrophobic properties. A well-known example of this phenomenon is the lotus leaf [2]. Hydrophobic surfaces with microscopic structures might reduce fluid resistance. This idea could be applied to the shark skin surfaces. In [3], the hydrophobicity of shark skin was estimated by measuring a contact angle between shark skin and a water droplet. However, this situation is quite different from that of actual swimming sharks. Hydrophobicity should be evaluated in an aqueous environment.
Therefore, in this study, an experimental system was developed to investigate interfacial hydrophobicity by using a bubble adhesion in an aqueous environment. To evaluate the wettability (or hydrophobicity) of a surface, the contact angle of a water droplet placed onto the target surface (in a gaseous environment) has been usually measured. When the contact angle is between 0° and 90°, the surface is considered hydrophilic, whereas the contact angle between 90° and 180° indicates hydrophobicity. In this study, in addition to the above-mentioned standard method, a different approach was employed in which a bubble was pushed to the target surface in a water pool. Then, the bubble was compressed and finally attached to the surface with a certain contact angle. After sufficient compression, the bubble was pulled until it left the surface. This method is employed because it characterizes the surface hydrophobicity in an aqueous environment, which is more appropriate to mimic the actual habitat of sharks. By comparing the contact angles measured in gaseous and aqueous environments, the role of microscopic structures can be elucidated in detail.
The experimental procedure began by submerging the tip of a syringe into a water pool and generating a bubble by pressing the plunger. The generated bubble had a radius of approximately 2 mm. Next, using a universal testing machine (Shimadzu AGX), the bubble was positioned just above the target surface. Then, the syringe was moved downward at a speed of 1 mm/min by 1.3 mm to press the bubble to the surface. After maintaining equilibrium for 10 seconds at rest, the syringe was moved upward by 4 mm at the same speed to detach the bubble. The contact angle measured during bubble compression is referred to as the receding contact angle, whereas the contact angle measured during detachment is termed the advancing contact angle. These contact angles were compared with the conventional contact angles obtained with the water-droplet method performed in a gaseous environment. By characterizing some polymer surfaces, we confirmed the validity of this measurement method. Finally, we measured the surface properties of actual shark skin using the developed method. The details of the measured result will be presented.
[1] Xia Pu, Guangji Li and Yunhong Liu, ChemBioEng Volume3, Issue1 2016.
[2] Mohammad Liravi, Hossein Pakzad, Ali Moosavi, and Ali Nouri-Borujerdi, Progress in Organic Coatings, Volume 140 2020.
[3] Yuehao Luo, Yufei Liu, Deyuan Zhang, and E. Y. K. NG, Journal of Mechanics in Medicine and Biology Vol. 14, No. 02 2014.
In general, when surfaces with fine rough structures contact a water droplet in atmospheric conditions, tiny air bubbles can be retained within the grooves, exhibiting hydrophobic properties. A well-known example of this phenomenon is the lotus leaf [2]. Hydrophobic surfaces with microscopic structures might reduce fluid resistance. This idea could be applied to the shark skin surfaces. In [3], the hydrophobicity of shark skin was estimated by measuring a contact angle between shark skin and a water droplet. However, this situation is quite different from that of actual swimming sharks. Hydrophobicity should be evaluated in an aqueous environment.
Therefore, in this study, an experimental system was developed to investigate interfacial hydrophobicity by using a bubble adhesion in an aqueous environment. To evaluate the wettability (or hydrophobicity) of a surface, the contact angle of a water droplet placed onto the target surface (in a gaseous environment) has been usually measured. When the contact angle is between 0° and 90°, the surface is considered hydrophilic, whereas the contact angle between 90° and 180° indicates hydrophobicity. In this study, in addition to the above-mentioned standard method, a different approach was employed in which a bubble was pushed to the target surface in a water pool. Then, the bubble was compressed and finally attached to the surface with a certain contact angle. After sufficient compression, the bubble was pulled until it left the surface. This method is employed because it characterizes the surface hydrophobicity in an aqueous environment, which is more appropriate to mimic the actual habitat of sharks. By comparing the contact angles measured in gaseous and aqueous environments, the role of microscopic structures can be elucidated in detail.
The experimental procedure began by submerging the tip of a syringe into a water pool and generating a bubble by pressing the plunger. The generated bubble had a radius of approximately 2 mm. Next, using a universal testing machine (Shimadzu AGX), the bubble was positioned just above the target surface. Then, the syringe was moved downward at a speed of 1 mm/min by 1.3 mm to press the bubble to the surface. After maintaining equilibrium for 10 seconds at rest, the syringe was moved upward by 4 mm at the same speed to detach the bubble. The contact angle measured during bubble compression is referred to as the receding contact angle, whereas the contact angle measured during detachment is termed the advancing contact angle. These contact angles were compared with the conventional contact angles obtained with the water-droplet method performed in a gaseous environment. By characterizing some polymer surfaces, we confirmed the validity of this measurement method. Finally, we measured the surface properties of actual shark skin using the developed method. The details of the measured result will be presented.
[1] Xia Pu, Guangji Li and Yunhong Liu, ChemBioEng Volume3, Issue1 2016.
[2] Mohammad Liravi, Hossein Pakzad, Ali Moosavi, and Ali Nouri-Borujerdi, Progress in Organic Coatings, Volume 140 2020.
[3] Yuehao Luo, Yufei Liu, Deyuan Zhang, and E. Y. K. NG, Journal of Mechanics in Medicine and Biology Vol. 14, No. 02 2014.