Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session A26: Wetting, Adhesion, and Tribology of Soft Interfaces IFocus Recordings Available
|
Hide Abstracts |
Sponsoring Units: DSOFT DPOLY GSNP Chair: Chelsea Davis, Purdue University Room: McCormick Place W-187B |
Monday, March 14, 2022 8:00AM - 8:12AM |
A26.00001: Soft surface layers control contact and slip of polyacrylamide hydrogels Md Mahmudul Hasan, Christopher L Johnson, Alison C Dunn Swollen gels with high water content >85% by mass tend to form softer surface layers due to the osmotic pressure mismatch between the bulk gel (Π>0) and that of the open bath (Π=0). These skin layers are locally swollen, and settle with a gradient profile of density that is homogeneous in the bulk, but vanishing at the surface. This experimental work uses broad probes in micro-indentation to characterize the contact mechanics of this gradient layer based on the evolving scaling of the indentation force F with depth F~dn. We find that the contact mechanics at the extreme of the surface scale like a polymer brush, the intermediate surface as a soft laminate, and finally the bulk as a homogeneous half-space. We also use finite-element simulations of indentation into slabs with controlled gradient surfaces to determine the most appropriate profile of the experimental data. The simulations explain how the stiffer bulk layer enforces the softer top layers to extend radially, which causes the observed increase the contact radius of the gradient samples. Finally, we show corresponding slip measurements, and connect the friction to the surface composition. These results facilitate the design the hydrogel with gradient layers, having a predictable contact and friction. |
Monday, March 14, 2022 8:12AM - 8:24AM |
A26.00002: Drop pinning on soft, lubricant-infused silicone elastomers ZHUOYUN CAI, Jonathan Pham Soft, slippery surfaces have gained increasing attention due to their wide range of potential applications, for example in biomaterials, self-cleaning, anti-fouling, liquid collection, and more. To prepare a soft, slippery surface, a crosslinked polymer network can be swelled with a compatible lubricant. However, an understanding of how swelling and crosslinking relate to slippery properties is still being developed. In this work, we focus on the influence of polymer network density (i.e. modulus) and degree of swelling on the water drop pinning force. Specifically, we study when a water drop sticks or slides on a vertical, silicone oil–swollen polydimethylsiloxane (PDMS) elastomer, where gravity drives the drop to slide. Our results indicate that the critical water drop volume for sliding is strongly controlled by the degree of swelling, even though a softer substrate may induce a larger wetting ridge that hinders drop sliding. In addition, we show that highly swollen surfaces can recover their slipperiness after they are rinsed with water. This is likely associated with regeneration of an oil-layer on the surface coming from the bulk substrate, demonstrating the durability of lubricant-swollen elastomers for practical uses. |
Monday, March 14, 2022 8:24AM - 8:36AM |
A26.00003: Contact lines in partial wetting: transition from stick-slip to steady sliding Bauyrzhan Primkulov, Amir A Pahlavan, Luis Cueto-Felgueroso, Ruben Juanes We study the motion of a viscous slug in a capillary tube, where one of its interfaces is in complete wetting while the other is in partial wetting conditions. When the partially wetting interface experiences strong pinning, the motion of the slug crosses over from stick-slip to steady sliding as its speed increases, reminiscing the dynamics of frictional sliding of solids. We characterize this transition using a combination of experiments, analytical arguments, and a computational model. Notably, this system can operate in either constant-rate or constant-force settings, which, in the limit of low displacement rates, allows probing the respective force-to-speed scaling relations by Raphael and de Gennes ( J. Chem. Phys. 1989) and Joanny and Robbins ( J. Chem. Phys. 1990). |
Monday, March 14, 2022 8:36AM - 9:12AM |
A26.00004: Friction of sparkling water drops on superhydrophobic surfaces Invited Speaker: Matilda Backholm When a carbonated water drop is placed on an extremely water repellent, superhydrophobic surface, a microscopically thin gas cushion of carbon dioxide is formed under the drop. These levitating “fizzy” drops have an extreme mobility similar to that of Leidenfrost drops moving on their own vapor when placed on a very hot surface. Probing the friction and dynamics of levitating drops will provide new insights into extreme anti-wetting. |
Monday, March 14, 2022 9:12AM - 9:24AM |
A26.00005: Contact angle hysteresis of a droplet on a hydrophobic, anisotropically curved surface Mingzhu Cui, Nadav Benhamou Goldfajn, Anthony D Dinsmore Our study is focused on the influence of solid surface geometry on contact angle hysteresis, specifically on the values of the advancing contact angle θA and receding contact angle θR. Recent studies show that when mm-sized spheres coated with polydimethylsiloxane (PDMS) were partially immersed in an air–water interface, θR decreased monotonically with increasing deviatoric curvature D, defined as half the difference between the two principal curvatures. θA remained unchanged. Here, we focus on a water droplet suspended from PDMS-coated flat glass plates and cylindrical glass rods with mm-scale diameters. Our droplet volume ranges from 2-24μL, with air-water-solid contact line diameters from about 1.7-2.7mm. In our experiment, we find the quasi-static θR decreases from about 84° to about 70° when D increases from 0 to 0.04/mm. The quasi-static θA decreases from about 103° to about 99° when D increases from 0 to 0.17/mm. Therefore, the contact angle hysteresis increases from about 19° to about 33° as D increases from 0 to 0.04/mm. In addition, we find that the contact line recedes more along the azimuthal direction than along the axial direction. This work may provide new insights into the origin of contact angle hysteresis and offer a route to calculating droplet shapes. |
Monday, March 14, 2022 9:24AM - 9:36AM |
A26.00006: Rubber friction: from steady sliding to stick slip and squeaking Gabriele Albertini, Adel Djellouli, David A Weitz, Katia Bertoldi Even though friction is responsible for keeping your car on the road, your coffee mug in your hand and on larger scales governs earthquakes, many aspects of it are still poorly understood. In particular, for soft materials moving on stiff ones there are still a lot of open questions. Here, we study both experimentally and numerically the frictional motion at the interface between a soft and a rigid body for driving velocities spanning four orders of magnitude. We find that, while the applied far field velocity is uniform, steady sliding at the frictional interface is not always dynamically stable. For driving velocities lower than a critical velocity we observe homogeneous steady sliding of the entire frictional interface. For larger velocities, however, slip is spatially localized along fronts. These fronts nucleate at the trailing edge of the soft sample, then propagate dynamically at speeds approaching the shear wave speed of the soft material. Finally, for driving velocities of the order of 1 m/s, sliding is associated with a distinct squeaking sound which frequency is correlated to the slip fronts. |
Monday, March 14, 2022 9:36AM - 9:48AM |
A26.00007: Temperature dependent soft wetting of polymer melts Krishnaroop Chaudhuri, Jonathan Pham Wetting studies on soft surfaces, though prolific, have been mostly limited to equilibrium cases at room temperature, often on crosslinked elastomers. In this work, we investigate the transient wetting characteristics of a polymer melt (poly n-butyl methacrylate) at temperatures much higher than its glass-transition. When a glycerol drop is placed on the polymer melt surface, an out-of-plane wetting ridge forms at the three-phase contact line. We use stylus profilometry to measure the height and the profile of the wetting ridge for different temperatures and times of contact between the drop and the melt. The wetting ridge growth rate is dependent on the system temperature and the time scale of the experiment. Using oscillatory rheology, scaling laws are developed to predict the time-dependent growth of the wetting ridge. We demonstrate that for a range of temperatures, both Rouse and reptation kinetics affect the rate of ridge growth over different timescales. Moreover, the shape profile of the wetting ridge is dictated by the time-dependent complex modulus, which is a departure from the constant shear modulus values often used for soft elastomers. However, the shape profile can be predicted by a simple consideration of the complex modulus within the context of current models. |
Monday, March 14, 2022 9:48AM - 10:00AM |
A26.00008: Rotation and Torque in Conforming and non-Conforming Lubricated Contacts Arash Kargar-Estahbanati, Bhargav Rallabandi Relative sliding motion between a rigid object and a soft substrate produces a symmetry-breaking deformation of the substrate, resulting in an elastohydrodynamic torque on the object. Here, we calculate this torque numerically for a wide range of sliding velocities in a two-dimensional configuration. This torque depends on the mechanical properties of the soft elastic substrate as well as the fluid properties and is analyzed for vanishingly thin and very thick substrates. In the limit of non-conforming contacts, corresponding to large velocities, we identify scaling relations for the elastohydrodynamic torque as a function of the speed. In the opposite limit of vanishingly small velocities (Hertzian contacts), the torque is dominated by the flow in a thin film of fluid that is entrained between the object and the elastic substrate. We show that for highly compliant substrates, the object behaves as it were rolling without slip on a solid frictional surface. We also discuss situations where the soft lubrication flow can be controlled by the application of an external torque. |
Monday, March 14, 2022 10:00AM - 10:12AM |
A26.00009: Quantification of adhesion force using a force tensiometer and influence of surrounding medium for a liquid droplet over a solid surface Palak Jain, Aleksey Baldygin, Prashant R Waghmare, Butunath Majhy Surface wetting and surface adhesion have gained major attention over the past few decades. This is primarily due to the plethora of applications based on liquid-solid interactions such as droplet transportation, superhydrophobic and self-cleaning surfaces to name a few. Recently, to characterize these surfaces, a few studies previously have measured the adhesion force using a force tensiometer and studied the modified surface properties of the surface based on adhesion force. The details of the technique and findings of these studies are still limited in the public domain. This paper outlines proposed necessary and precise steps to quantify the magnitude of adhesion force directly and the parameters that affect the magnitude of adhesion force, such as droplet volume and compression value are also taken into account. Moreover, this work takes into consideration the effect of the surrounding medium on adhesion force. This aspect is achieved by performing experiments under a liquid medium and observing the influence of other forces like buoyancy. |
Monday, March 14, 2022 10:12AM - 10:24AM |
A26.00010: Measuring elastocapillary dissipation in droplets moving along soft elastomer films Hamza K Khattak, Stefan Karpitschka, Jacco H Snoeijer, Kari Dalnoki-Veress When a droplet interacts with a soft surface, it can deform the material it contacts. This property leads to a plethora of unique physical phenomena with applications in fields ranging from water collection to surface sensing. We explore droplet dynamics on soft materials using a micropipette-based technique to simultaneously image, and measure the forces on, a microscopic droplet dragged along the surface of a soft elastomeric polydimethylsiloxane (PDMS) film. By changing the thickness of the elastomer film, we can control the compliance of the substrate independent of surface chemistry. We are also able to control and measure the presence of uncrosslinked PDMS chains in our system. We model the dynamics of the droplet-substrate interaction and expect dissipation to scale with the size of the capillary deformation. We find agreement between our model and experimental results. |
Monday, March 14, 2022 10:24AM - 10:36AM |
A26.00011: X-ray microscopy visualizes the indentation of colloids into soft elastic solids Joonwoo Jeong, Jae Kwan Im Soft matter at small length scales where the interfacial energy could be predominant often has curved interfaces to minimize the interfacial area, causing hardship in the optical-microscopy investigation. X-ray imaging seeing through the specimen in situ can overcome this hurdle, even requiring no specific sample treatment and environment. Using X-ray microscopy, we examine the adhesive contact between a silica sphere and a silicone elastomer substrate. Varying the particle sizes and Young's modulus of substrates, we confirm the crossover behavior between elastic and liquid-like substrates; The model combining the JKR theory and the surface stress of the substrate well describes the data. To our interest, the X-ray allows a direct measurement of the ridge height, in addition to the indentation depth and contact radius. The ridge height is linearly proportional to the indentation depth, not only in the elastic regime as expected by the JKR theory but also in the liquid-like regime. |
Monday, March 14, 2022 10:36AM - 10:48AM |
A26.00012: The role of crosslinking density in surface stress and surface energy of soft solids Qin Xu, Weiwei ZHAO Surface stress and surface energy are two fundamental parameters that determine the surface properties of any material. While it is commonly believed that the surface stress and surface energy of liquids are identical, the relationship between the two parameters in soft polymeric gels remains debatable. In this work, we measured the surface stress and surface energy of soft silicone gels with varying crosslinking densities in soft wetting experiments. Above a critical crosslink density, $k_0$, the surface stress is found to increase significantly with crosslinking density while the surface energy, by contrast, remains unchanged. In this regime, we can estimate a non-zero surface elastic modulus that also increases with the ratio of crosslinkers. By comparing the surface mechanics of the soft gels to their bulk rheology, the surface properties near the critical density $k_0$ are found to be closely related to the underlying percolation transition of the polymer networks. |
Monday, March 14, 2022 10:48AM - 11:00AM |
A26.00013: Molecular mechanisms of self-mated hydrogel friction Jan Mees, Lars Pastewka The friction coefficient of self-mated hydrogel systems is velocity independent at slow sliding speeds and transitions to a velocity dependent regime at a mesh size dependent velocity. Whilst the speed independent regime has been attributed to the thermal fluctuations, above the transition the increase of the friction coefficient has been attributed to either hydrodynamic lubrication or non-equilibrium polymer dynamics. We have developed a mesoscopic model based on bead-spring polymer networks, which is able to reproduce the speed-dependent frictional behavior. Our model qualitatively reproduces the experimentally observed collapse of friction curves achieved by rescaling the sliding speed. By using implicit solvent, we are able disentangle the role played by hydrodynamic interactions and polymer interactions in the frictional behavior of self-mated hydrogels. We further present experimental data confirming our findings from simulations that the coefficient of friction increases with mesh size for hydrogels with highly crosslinked interfaces. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700