Forming flower-like fingers with fluid instabilities

Anyone who has dropped watercolor paint onto a porous sheet of paper has seen how the paint spreads radially in fingers, a process governed by the Rayleigh-Taylor and Saffman-Taylor instabilities. In certain conditions, the fluid fingers widen, creating a beautiful flower-like effect. Studying this behavior can shed light on important phenomena like coral formation, oil spill cleanup, or cell growth.

Vanarse et al. investigated this flower formation at the interface of a solvent and non-solvent. They used cellulose acetate (CA) and dimethylformamide (DMF) solution mixed with water to investigate the instabilities and the key parameters.

“The combination of CA and DMF enables the formation of thin films due to their compatibility and the ability of DMF to quickly mix with the water, leaving behind a film of cellulose acetate,” said author Vinod Babasaheb Vanarse.

The team utilized sterilized glass substrates coated in the CA-DMF solution. They then dispensed drops of water, a non-solvent, which caused a flower-like pattern to form on the slides.

“The elongation and lateral growth of the fingers are influenced by a complex interplay of capillary and osmotic pressure gradients, viscous drag force, and solutal Marangoni effects,” said author Dipankar Bandyopadhyay. “The viscosity and density differences between water and the soft-deformable porous layer play a crucial role, ensuring that larger viscosity and density disparities result in smaller fingers with a larger number.”

In the future, the team plans to continue refining their experimentation and introduce numerical simulations to complement the experimental studies.

Source: “Coupled instability modes at a solvent/non-solvent interface to decorate cellulose acetate flowers,” by Vinod Babasaheb Vanarse, Siddharth Thakur, Abir Ghosh, Prathu Raja Parmar, and Dipankar Bandyopadhyay, Physics of Fluids (2024). The article can be accessed at https://doi.org/10.1063/5.0188222 .