When joints open while the adhesive is still rather liquid, structures like shown in the pictures can be detected. These structure are very helpful in failure analysis as the prove the rather low viscosity of the adhesive in the moment of the failure. We called them honey on toast structure for the lack of a better term as they resemble the structure of a honey covered toast after falling on the wrong side. But is there somewhere a real/better term for this structure? Are there publications about the dependency of the structures on adhesive thickness and viscosity?
I do not know any names for this. I once spent some time looking at the patterns formed when a pressure-sensitive adhesive was pulled apart (approximately normally) as at a peel front. I think that they looked quite similar (though I cannot enlarge your pictures). It seemed to me that they might be fractal-type patterns, but we never managed to analyse them fully.
Meanwhile we found some literature and scientific names for the structures. The structures resembling natural valeys/rivers (my first 3 pictures) can be addressed as: fingering, viscoelastic fingering or air fingers. To my understanding, they are no fractals, as the self-similarity stops at an early scale. The honeycomb-like structures (last picture - red) are adressed as cavitation in the literature.
According to literature and some own experiments, fingers and caviations depend on the following aspects: In the case of air (or an other low viscosity material) beeing present, factors that facilitate the flow of the adhesive lead to fingers. Conversely, factors that inhibit the flow of the adhesive lead to small fingers and promote the appearance of honeycomb structures (cavitation). Fingers can only occour if air is present. Fingers are promoted by a low internal friction (viscosity), a slow speed of separation of the parts and a high thickness of the adhesive layer. If no air can penetrate into the adhesive, only cavitation are formed. Cavitations originate from unavoidable preexisting bubble nuclei in the range of a few µm that enlarge when pulling forces are applied
Amar MB, Bonn D, 2005. Fingering instabilities in adhesive failure. Physica D: Nonlinear Phenomena 209, 1–16. DOI: 10.1016/j.physd.2005.07.002.
Nase J, Derks D, Lindner A, 2011. Dynamic evolution of fingering patterns in a lifted Hele–Shaw cell. Physics of Fluids 23, 123101. DOI: 10.1063/1.3659140.
Poivet S, Nallet F, Gay C, Fabre P, 2003. Cavitation-induced force transition in confined viscous liquids under traction. EPL (Europhysics Letters) 62, 244.
Poivet S, Nallet F, Gay C, Teisseire J, Fabre P, 2004. Force response of a viscous liquid in a probe-tack geometry: fingering versus cavitation. The European physical journal. E, Soft matter 15, 97–116. DOI: 10.1140/epje/i2004-10040-2.
Saffman PG, Taylor G, 1958. The Penetration of a Fluid into a Porous Medium or Hele-Shaw Cell Containing a More Viscous Liquid. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 245, 312–329. DOI: 10.1098/rspa.1958.0085.
Tanguy F, Nicoli M, Lindner A, Creton C, 2014. Quantitative analysis of the debonding structure of soft adhesives. The European physical journal. E, Soft matter 37, 3. DOI: 10.1140/epje/i2014-14003-8.
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