Crack patterns of black ink film: sumi-wari

Shimokawa, M, Goehring, L ORCID: https://orcid.org/0000-0002-3858-7295, Kinoshita, A, Pauchard, L and Sakaguchi, H, 2026. Crack patterns of black ink film: sumi-wari. Soft Matter. ISSN 1744-683X

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Abstract

Local variations in surface tension can induce complex fracture dynamics in thin interfacial films. Here, we investigate the fracture patterns that emerge when a localized surface-tension perturbation is applied to a sumi film supported on a water–glycerol subphase. Sumi is a traditional Japanese carbon black ink, and this process, referred to as sumi-wari, produces aesthetically pleasing, star-shaped crack patterns with multiple spikes radiating from the perturbation site. The number of crack spikes increases with the viscosity of the subphase, controlled here by the addition of glycerol. Atomic force microscopy measurements reveal that the effective stiffness of the sumi film decreases as glycerol concentration increases. This suggests a strong coupling between the subphase properties and the mechanics of the sumi film. To capture the dynamics of sumi-wari, a phenomenological model is outlined, based on an overdamped equation of motion for particles connected by breakable springs. Numerical simulations reproduce both the morphology and the experimental trends of sumi-wari: the number of cracks and their temporal evolution depend on the spring stiffness, mirroring the behavior observed for subphases with different viscosities. These findings demonstrate how the interplay between surface-tension gradients, subphase properties, and film mechanics governs local fracture and pattern formation in fluid–supported thin films.

Item Type:	Journal article
Publication Title:	Soft Matter
Creators:	Shimokawa, M., Goehring, L., Kinoshita, A., Pauchard, L. and Sakaguchi, H.
Publisher:	Royal Society of Chemistry (RSC)
Date:	17 March 2026
ISSN:	1744-683X
Identifiers:	Number Type 10.1039/D5SM01281C DOI 2595192 Other
Divisions:	Schools > School of Science and Technology
Record created by:	Melissa Cornwell
Date Added:	30 Mar 2026 09:38
Last Modified:	30 Mar 2026 09:38
URI:	https://irep.ntu.ac.uk/id/eprint/55484

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