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Recent experiments reveal that adhesive interactions can play a key role in causing surface instability in soft lubrication. Instances of instability include fluid entrapment in isolated pockets upon a soft sphere’s normal contact with a hard substrate and surface wrinkling of a soft substrate as a hard sphere slides across it. These phenomena underscore a substantial distinction between hard and soft lubrication. They are of paramount importance from a fundamental standpoint, providing an entirely new explanation for the transition mechanism from elasto-hydrodynamic to the mixed lubrication regimes. Here, we introduce a new theory to elucidate these observations. Our theory modifies the Reynolds elasto-hydrodynamic equation by incorporating adhesive interaction across the fluid layer, investigating the interplay between adhesion, fluid flow and elastic instability. Our analysis proposes the addition of a new dimensionless parameter in lubrication theory, that compares the stiffness of the adhesive interaction to that of the substrate. When this parameter exceeds unity, the soft solid surface exhibits instability to small perturba- tions in its shape. In mathematical terms, the Reynolds equation undergoes a transition from a nonlinear diffusion equation to a nonlinear wave equation at this critical point. Post-transition, the diffusivity of the nonlinear diffusion equation turns negative, rendering the problem ill- posed. We investigate the transition using the method of characteristics and present an exact analytic solution. This solution offers insights into the occurrence of a vanishing liquid film thickness at specific locations, resulting in dry contact—initiating transition to mixed lubrication. 

Traditional PhD training in STEM fields places a strong emphasis on developing doctoral students' academic skills, encompassing research, academic writing, as well as sharing of knowledge through publications and conference presentations, etc. However, with the ever evolving expectations of graduate training, particularly in applied fields, the demand for PhD has transcended the confines of academia. For instance, nearly 90% of engineering PhDs will not enter academia, which underscores the discrepancy between the current PhD training programs and the preparation of students for future careers. To better support doctoral students especially for those who intend to pursue positions in industry including corporate R&D labs, national labs, defense organizations, healthcare institutes, etc., Lehigh University launched an innovative program called Pasteur Partners PhD (P3) specifically for the training of such doctoral students. It is a student-centered doctoral training program based on use-inspired research in partnership with industry. A preliminary evaluation of the P3 program, which was developed with support from NSF’s IGE program, revealed that students benefited significantly from gaining practical skills through industry involvement such as co-advising, resulting in a clearer understanding of how the industry operates, which, in turn, enhanced their employability in the industry [1]. The University administration also provided significant support for the program. However, a broader implementation of P3 encountered challenges and hesitancy from faculty members. Mostly the senior faculty who already had preexisting connections with industry and junior faculty from certain departments were more receptive to joining the P3 program than others. Could this be a result of the prevailing emphasis of the graduate education system on research output (publications) rather than the training of students for their subsequent careers? What other reasons could there be for the faculty’s lack of enthusiasm for the training of their PhD students following P3 track? To answer above questions and examine the challenges and obstacles that the faculty members feel for student centered doctoral training from an institutional and system perspective, we are conducting a survey specifically targeting faculty members in STEM fields. It seeks to comprehensively understand faculty members’ perspective on the primary objectives of doctoral training within different STEM fields. By exploring these objectives, the survey aims to uncover how they vary across disciplines and what faculty members perceive as the most significant goals in their areas of expertise. Moreover, the survey is designed to shed light on the challenges and hurdles faced by faculty members in their pursuit of these training objectives. Faculty participants are encouraged to identify and articulate the specific obstacles they encounter, whether they pertain to institutional constraints, resource limitations, demands of perceived professional success or other factors that impede the realization of these goals. In addition, the survey takes a close look at the resources that faculty members believe would be beneficial in addressing these challenges and improving the effectiveness of doctoral training. This insight is essential for designing support systems that can empower faculty to contribute to the training of doctoral workforce for the benefit of society at large. The survey seeks to gain valuable perspectives on the qualities and skills considered essential for the success of PhD students. These insights will inform curriculum development and help prepare students better for a wider range of career paths. The results of the survey, currently underway, are presented. 

Biology is replete with examples, at length scales ranging from the molecular (ligand–receptor binding) to the mesoscopic scale (wing arresting structures on dragonflies) where shape-complementary surfaces are used to control interfacial mechanical properties such as adhesion, friction, and contact compliance. Related bio-inspired and biomimetic structures have been used to achieve unique interfacial properties such as friction and adhesion enhancement, directional and switchable properties. The ability to tune friction by altering surface structures offers advantages in various fields, such as soft robotics and tire manufacturing. Here, we present a study of friction between polydimethylsiloxane (PDMS) samples with surfaces patterned with pillar-arrays. When brought in contact with each other the two samples spontaneously produce a Moire´ pattern that can also be represented as an array of interfacial dislocations that depends on interfacial misorientation and lattice spacing. Misorientation alone produces an array of screw dislocations, while lattice mismatch alone produces an array of edge dislocations. Relative sliding motion is accompanied by interfacial glide of these patterns. The frictional force resisting dislocation glide arises from periodic single pillar–pillar contact and sliding. We study the behavior of pillar–pillar contact with larger (millimeter scale) pillar samples. Inter-pillar interaction measurements are combined with a geometric model for relative sliding to calculate frictional stress that is in good agreement with experiments. 

Insects and small animals often utilize structured surfaces to create friction during their movements. These surfaces typically consist of pillar-like fibrils that interact with a counter surface. Understanding the mechanical interaction between such surfaces is crucial for designing structured surfaces for engineering applications. In the first part of our study, we examined friction between poly(dimethylsiloxane) (PDMS) samples with surfaces patterned with pillar-arrays. We observed that sliding between these surfaces occurs through the interfacial glide of dislocation structures. The frictional force that resists this dislocation glide is a result of periodic single pillar-pillar contact and sliding. Hence, comprehending the intricate interaction between individual pillar contacts is a fundamental prerequisite for accurately modeling the friction behavior of the pillar array. In this second part of the study, we thoroughly investigated the contact interaction between two pillars located on opposite sides of an interface, with different lateral and vertical offsets. We conducted experiments using PDMS pillars to measure both the reaction shear and normal forces. Contact interaction between pillars was then studied using finite element (FE) simulations with the Coulomb friction model, which yielded results that aligned well with the experimental data. Our result offers a fundamental solution for comprehending how fibrillar surfaces contact and interact during sliding, which has broad applications in both natural and artificial surfaces. 

Viral infection usually begins with adhesion between the viral particle and viral receptors displayed on the cell membrane. The exterior surface of the cell membrane is typically coated with a brush-like layer of molecules, the glycocalyx, that the viruses need to penetrate. Although there is extensive literature on the biomechanics of virus−cell adhesion, much of it is based on continuum-level models that do not address the question of how virus/cell-membrane adhesion occurs through the glycocalyx. In this work, we present a simulation study of the penetration mechanism. Using a coarse-grained molecular model, we study the force-driven and di"usive penetration of a brush-like glycocalyx by viral particles. For force-driven penetration, we find that viral particles smaller than the spacing of molecules in the brush reach the membrane surface readily. For a given maximum force, viral particles larger than the minimum spacing of brush molecules arrest at some distance from the membrane, governed by the balance of elastic and applied forces. For the di"usive case, we find that weak but multivalent attraction between the glycocalyx molecules and the virus e"ectively leads to its engulfment by the glycocalyx. Our finding provides potential guidance for developing glycocalyx-targeting drugs and therapies by understanding how virus−cell adhesion works. 

Adhesives typically fall into two categories: those that have high but irreversible adhesion strength due to the formation of covalent bonds at the interface and are slow to deploy, and others that are fast to deploy and the adhesion is reversible but weak in strength due to formation of noncovalent bonds. Synergizing the advantages from both categories remains challenging but pivotal for the development of the next generation of wound dressing adhesives. Here, we report a fast and reversible adhesive consisting of dynamic boronic ester covalent bonds, formed between poly(vinyl alcohol) (PVA) and boric acid (BA) for potential use as a wound dressing adhesive. Mechanical testing shows that the adhesive film has strength in shear of 61 N/cm 2 and transcutaneous adhesive strength of 511 N/cm 2 , generated within 2 min of application. Yet the film can be effortlessly debonded when exposed to excess water. The mechanical properties of PVA/BA adhesives are tunable by varying the cross-linking density. Within seconds of activation by water, the surface boronic ester bonds in the PVA/BA film undergo fast debonding and instant softening, leading to conformal contact with the adherends and reformation of the boronic ester bonds at the interface. Meanwhile, the bulk film remains dehydrated to offer efficient load transmission, which is important to achieve strong adhesion without delamination at the interface. Whether the substrate surface is smooth (e.g., glass) or rough (e.g., hairy mouse skin), PVA/BA adhesives demonstrate superior adhesion compared to the most widely used topical skin adhesive in clinical medicine, Dermabond.