Cuttlefish’s Ink Patterns Generated by Three Liquids Mixing and Jetting

Yu-Ting Lin^1*, I-Hsuan Chen², Patricia J. Yang¹

¹Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, Taiwan
²Program of Life Sciences and Medicine, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Yu-Ting Lin, email:yuting0121.dry@gmail.com

When cuttlefish face danger, they eject ink as a defense. They mix ink, mucus, and saltwater and jet six types of ink patterns through their funnel. Previous studies show cuttlefish ink's properties and ink ejection's chemical defense mechanisms. However, the physics of cuttlefish ink ejection and the properties of cuttlefish mucus are still poorly understood. Here, we investigate how cuttlefish mix these three liquids (ink, mucus, saltwater) and how the mixture displays the different shapes in the sea. In our study, we observed 10 mm-body-length cuttlefish, Sepia pharaonic, jetting ink, collected mucus with the tubeless siphon effect, measured the viscosity of mucus, and mimicked the process with a tabletop device. From the animal observation, we classified ink patterns into four types: blob, vortex ring, rope, and puff. The blob is a rounded ink mass roughly the same size as a cuttlefish's body. The vortex ring is shaped like a donut with a clear hole in the center. The rope takes on a thin, elongated form, appearing as a line of ink. Lastly, the puff consists of multiple small vortices through which the ink rapidly mixes with the surrounding saltwater, eventually dispersing evenly throughout. The first three types have a clear boundary with salt water, and the boundary does not disperse in two hours. The puff disperses in the saltwater in 1 sec. The Reynolds numbers for the above four types of ink patterns fall within the following ranges: 319 ~ 1438, 1035 ~ 1245, 603 ~ 868, and 245 ~ 2786, respectively. We also found that cuttlefish jet transparent mucus and the bubbles don’t break in the mucus, which means the mucus is a polymer. The viscosity of mucus is 627 to 4.7 mPa⋅s at a shear rate of 8.13 to 2000 1/s. Our findings would advance liquid mixing methods applied in the pharmaceutical industry and inkjet technology. We also created a new method to collect mucus from saltwater.

Keywords: Vortex ring, Formation time, Laminar flow, Turbulent flow, Sepia pharaonic