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Kim AR, Mitra SK, Zhao B. Unravelling soft interfaces: Visualization of gel ridges. J Colloid Interface Sci 2024; 676:1109-1117. [PMID: 39079274 DOI: 10.1016/j.jcis.2024.07.112] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2024] [Revised: 06/27/2024] [Accepted: 07/13/2024] [Indexed: 09/19/2024]
Abstract
HYPOTHESIS Soft materials, particularly elastomers, are extensively studied, but investigations into purely soft gel contact systems are limited due to their complex dual phases consisting of polymer and free liquids. While Dual Wavelength-Reflection Interference Confocal Microscopy (DW-RICM) is effective for noninvasively visualizing interfaces from a bottom view, it faces challenges in gel studies due to close refractive indices of polymeric networks and free liquids. We hypothesize that modulating the refractive index of soft gels using nanoparticles (NPs) enhances the visualization of contact zone beneath the free surface, providing insights into the configuration of phase-separated free oil within gel-on-gel contact systems. EXPERIMENTS Gel-on-gel contact systems were fabricated using immiscible organogels and hydrogels. Titanium dioxide (TiO2) NPs were introduced into the organogel to modulate refractive indices. Given the lack of prior studies on the hidden contact zone between gels, various techniques, including DW-RICM, side-view imaging, and inverted optical microscopy, were employed to observe and validate our findings. Comparative analyses were conducted with elastomer-on-rigid, elastomer-on-gel, and gel-on-rigid contact systems. FINDINGS Our investigation demonstrated that a minimal amount of TiO2 NPs effectively delineates the direct contact radius between organogel polymeric networks and hydrogel surfaces. Comparative experiments showed that TiO2 addition did not alter the gels' mechanical and surface properties but significantly enhanced information on gel contact deformation. This enhanced visualization technique has the potential to advance our understanding of adhesive contacts in gels, providing valuable insights into interface phenomena involving biological soft tissues and cells.
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Affiliation(s)
- A-Reum Kim
- Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada; Department of Mechanical & Mechatronics Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
| | - Sushanta K Mitra
- Department of Mechanical & Mechatronics Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
| | - Boxin Zhao
- Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
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Rahat SA, Chaudhuri K, Pham JT. Capillary detachment of a microparticle from a liquid-liquid interface. SOFT MATTER 2023; 19:6247-6254. [PMID: 37555264 DOI: 10.1039/d3sm00470h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/10/2023]
Abstract
The attachment and detachment of microparticles at a liquid-liquid interface are common in many material systems, from Pickering emulsions and colloidal assemblies to capillary suspensions. Properties of these systems rely on how the particles interact with the liquid-liquid interface, including the detachment process. In this study, we simultaneously measure the capillary detachment force of a microparticle from a liquid-liquid interface and visualize the shape of the meniscus by combining colloidal probe microscopy and confocal microscopy. The capillary behavior is studied on both untreated (hydrophilic) and fluorinated (hydrophobic) glass microparticles. The measured force data show good agreement with theoretical calculations based on the extracted geometric parameters from confocal images of the capillary bridge. It is also evident that contact line pinning is an important aspect of detachment for both untreated and fluorinated particles.
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Affiliation(s)
- Sazzadul A Rahat
- Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221, USA.
| | - Krishnaroop Chaudhuri
- Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, USA
| | - Jonathan T Pham
- Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221, USA.
- Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, USA
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Glover JD, Yang X, Long R, Pham JT. Creasing in microscale, soft static friction. Nat Commun 2023; 14:2362. [PMID: 37095110 PMCID: PMC10126204 DOI: 10.1038/s41467-023-38091-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2023] [Accepted: 04/17/2023] [Indexed: 04/26/2023] Open
Abstract
Utilizing colloidal probe, lateral force microscopy and simultaneous confocal microscopy, combined with finite element analysis, we investigate how a microparticle starts moving laterally on a soft, adhesive surface. We find that the surface can form a self-contacting crease at the leading front, which results from a buildup of compressive stress. Experimentally, creases are observed on substrates that exhibit either high or low adhesion when measured in the normal direction, motivating the use of simulations to consider the role of adhesion energy and interfacial strength. Our simulations illustrate that the interfacial strength plays a dominating role in the nucleation of a crease. After the crease forms, it progresses through the contact zone in a Schallamach wave-like fashion. Interestingly, our results suggest that this Schallamach wave-like motion is facilitated by free slip at the adhesive, self-contacting interface within the crease.
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Affiliation(s)
- Justin D Glover
- Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY, 40506, USA
| | - Xingwei Yang
- Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO, 80309, USA
| | - Rong Long
- Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO, 80309, USA
| | - Jonathan T Pham
- Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY, 40506, USA.
- Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, 45221, USA.
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Dong H, Moyle N, Wu H, Khripin CY, Hui CY, Jagota A. The transition from Elasto-Hydrodynamic to Mixed Regimes in Lubricated Friction of Soft Solid Surfaces. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2023; 35:e2211044. [PMID: 36779906 DOI: 10.1002/adma.202211044] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/26/2022] [Revised: 01/29/2023] [Indexed: 05/17/2023]
Abstract
Lubricated contacts in soft materials are common in various engineering and natural settings, such as tires, haptic applications, contact lenses, and the fabrication of soft electronic devices. Two major regimes are elasto-hydrodynamic lubrication (EHL), in which solid surfaces are fully separated by a fluid film, and mixed lubrication (ML), in which there is partial solid-to-solid contact. The transition between these regimes governs the minimum sliding friction achievable and is thus very important. Generally, the transition from EHL to ML regimes is believed to occur when the thickness of the lubricant layer is comparable with the amplitude of surface roughness. Here, it is reported that in lubricated sliding experiments on smooth, soft, poly(dimethylsiloxane) substrates, the transition can occur when the thickness of the liquid layer is much larger than the height of the asperities. Direct visualization of the "contact" region shows that the transition corresponds to the formation of wave-like surface wrinkles at the leading contact edge and associated instabilities at the trailing contact edge, which are believed to trigger the transition to the mixed regime. These results change the understanding of what governs the important EHL-ML transition in the lubricated sliding of soft solids.
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Affiliation(s)
- Hao Dong
- Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA, 18015, USA
| | - Nichole Moyle
- Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA, 18015, USA
| | - Haibin Wu
- Department of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, 14853, USA
| | | | - Chung-Yuen Hui
- Department of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, 14853, USA
| | - Anand Jagota
- Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA, 18015, USA
- Department of Bioengineering, Lehigh University, Bethlehem, PA, 18015, USA
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Kim AR, Mitra SK, Zhao B. Capillary pressure mediated long-term dynamics of thin soft films. J Colloid Interface Sci 2022; 628:788-797. [PMID: 36029593 DOI: 10.1016/j.jcis.2022.08.075] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2022] [Revised: 07/24/2022] [Accepted: 08/11/2022] [Indexed: 10/15/2022]
Abstract
HYPOTHESIS The conventional solid-solid contact is well studied in the literature. However, a number of practical applications, such as adhesive patches and biomimetic surfaces, require a much deeper understanding of soft contact where there is a distinct time-dependent adhesion behavior due to the dual-phase structure (solids and liquids). To understand this, currently existing solid-solid contact behavior is extrapolated to soft contact, wherein the size-effect of the gel film and the preload are typically neglected. When introducing the finite-size effect and preload, gels could experience distinctive long-term contact dynamics in contact with another material. EXPERIMENTS We reconstruct the evolving surface profile of the gel films intercalated between a glass sphere and glass slide using dual wavelength-reflection interference contrast microscopy. The macro-sized glass sphere compresses the gel. The indentation depth is comparable to the gel film thickness, wherein the conventional contact theories are inapplicable. FINDINGS The gel surface experiences two deformation stages. The natural preload and elastic force develop the contact area in the early state. In the later state, the viscous free molecules of the gel develop the ridge. We discover that the residual surface stress relaxes over 85 hr. Our findings on the long-term gel deformation provide a new perspective on soft adhesion, from developing soft adhesives to understanding biological tissues.
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Affiliation(s)
- A-Reum Kim
- Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
| | - Sushanta K Mitra
- Department of Mechanical & Mechatronics Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
| | - Boxin Zhao
- Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
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Darby DR, Cai Z, Mason CR, Pham JT. Modulus and adhesion of Sylgard 184, Solaris, and Ecoflex 00‐30 silicone elastomers with varied mixing ratios. J Appl Polym Sci 2022. [DOI: 10.1002/app.52412] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Affiliation(s)
- Daniel R. Darby
- Department of Chemical and Materials Engineering University of Kentucky Lexington Kentucky USA
| | - Zhuoyun Cai
- Department of Chemical and Materials Engineering University of Kentucky Lexington Kentucky USA
| | - Christopher R. Mason
- Department of Chemical and Materials Engineering University of Kentucky Lexington Kentucky USA
| | - Jonathan T. Pham
- Department of Chemical and Materials Engineering University of Kentucky Lexington Kentucky USA
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