Adhesives with a thickness smaller than the fractocohesive length enhance adhesion.
basic_science · Level V
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- Record sourced from PubMed, PMID 42388036.
- Also identified by DOI 10.1039/d6sm00382f.
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Abstract
When two materials are adhered by an adhesive, two length scales emerge: the adhesive thickness and the adhesive fractocohesive length. The former is a geometric length, and the latter is a material length, defined as the fracture toughness divided by the work of rupture of the bulk adhesive. The fractocohesive length scales with the size of the process zone around a crack tip. In this paper, we show that the ratio of the adhesive thickness to the fractocohesive length, <i>h</i>/<i>l</i><sub>f</sub>, can control adhesion. When <i>h</i>/<i>l</i><sub>f</sub> > 1, the process zone is confined in the adhesive layer. The debonding elicits dissipation in the adhesive, and the adhesion energy is on a similar level to the fracture toughness of the adhesive. When <i>h</i>/<i>l</i><sub>f</sub> < 1, the process zone extends beyond the adhesive layer to the adherends. The debonding elicits dissipation from both the adhesive and the adherends, and the adhesion energy is amplified on a similar level to the fracture toughness of the adherend. We illustrate this mechanism by bonding tough alginate-polyacrylamide hydrogels using brittle alginate and polyacrylamide adhesives with varied <i>h</i>/<i>l</i><sub>f</sub>. We also study debonding speeds on <i>h</i>/<i>l</i><sub>f</sub> and adhesion. We further explore adhesion behaviors with non-uniform thickness adhesives. These experiments support our proposed mechanism.