Small scale contact between a soft, liquid-coated layer and a stiff surface is common in many situations, from synovial fluid on articular cartilage to adhesives in humid environments. Moreover, many model studies on soft adhesive contacts are conducted with soft silicone elastomers, which possess uncrosslinked liquid molecules ( i.e. silicone oil) when the modulus is low. We investigate how the thickness of a silicone oil layer on a soft substrate relates to the indentation depth of glass microspheres in contact with crosslinked PDMS, which have a modulus of <10 kPa. The particles indent into the underlying substrate more as a function of decreasing oil layer thickness. This is due to the presence of the liquid layer at the surface that causes capillary forces to pull down on the particle. A simple model that balances the capillary force of the oil layer and the minimal particle–substrate adhesion with the elastic and surface tension forces from the substrate is proposed to predict the particle indentation depth.
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Marangoni-induced reversal of meniscus-climbing microdroplets
Small water droplets or particles located at an oil meniscus typically climb the meniscus due to unbalanced capillary forces. Here, we introduce a size-dependent reversal of this meniscus-climbing behavior, where upon cooling of the underlying substrate, droplets of different sizes concurrently ascend and descend the meniscus. We show that microscopic Marangoni convection cells within the oil meniscus are responsible for this phenomenon. While dynamics of relatively larger water microdroplets are still dominated by unbalanced capillary forces and hence ascend the meniscus, smaller droplets are carried by the surface flow and consequently descend the meniscus. We further demonstrate that the magnitude and direction of the convection cells depend on the meniscus geometry and the substrate temperature and introduce a modified Marangoni number that well predicts their strength. Our findings provide a new approach to manipulating droplets on a liquid meniscus that could have applications in material self-assembly, biological sensing and testing, or phase change heat transfer.
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- Award ID(s):
- 1856722
- PAR ID:
- 10429812
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 19
- Issue:
- 4
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 625 to 633
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2SM00979J
