Prof. Koshi Adachi (Tohoku University, Japan)

Prof. Dr. Ramin Aghababaei (Aarhus University, Denmark)

Title:
Contact Mechanics of Soft Polymeric Shells: A Path to Tunable Friction and Adhesion
Abstract:
Friction and wear are not inherent material constants but system-level responses shaped by complex interactions among geometry, material properties, and loading conditions. In this presentation, I will discuss recent advances in the contact mechanics of soft polymeric shell structures and their implications for engineering friction and adhesion in soft matter systems. By exploiting the interplay between structural instabilities and local tribological behavior, we demonstrate how surface roughness and contact mechanics can be actively modulated. This includes the emergence of unconventional behaviors such as a nonmonotonic relationship between applied load and contact area, providing insights into the breakdown of classical Hertzian contact theory. These findings open new avenues for designing surfaces with tunable friction and adhesion—key to applications ranging from soft robotics to biomedical devices.

Prof. Daniele Dini (Imperial College of London, UK)

Prof. Enrico Gnecco (Jagiellonian University, Poland)

Title:
Early-Stage Wear of Polymeric and Layered Materials at the Nanoscale
Abstract:
Atomic force microscopy (AFM) is a key tool in the characterization of early-stage wear at the nanoscale. Here, we will discuss this possibility for standard polymeric and layered materials. When a polystyrene surface is repeatedly scraped by AFM a significant outcome is the extrusion and release of nanoplastics out of the crests of regular ripple patterns caused by a scan-induced crazing mechanism [1]. The ripple formation is well reproduced by an original model combining the viscoplastic indentation and elastic shear exerted by the AFM tip on the compliant polymer surface [2]. While generating the ripples, the tips undergoes a stick-slip motion. This is also the case for multilayer MoS2 scraped by diamond probes, although the nanowear mechanism develops very differently on this brittle material. First, MoS2 flakes are exfoliated as a result of surface cracks propagated from the wear track. Second, the stick-slip has a characteristic length scale well below the linear size of the flakes. It is also irregular, with force drops characteristic of avalanche dynamics. The nanowear tests on MoS2 are corroborated by MD simulations that not only confirm the stick-slip mechanism, but also relate it to atomic-scale features (e.g. local amorphization) not detectable by AFM. Considering the environmental and health impact of nanoplastic pollution and, respectively, the relevance of layered materials for nanomachining and/or solid lubrication, our ongoing work will hopefully stimulate new collaborations on such an important but rather unexplored research topic.
[1] J. Hennig et al., Nucleation and detachment of polystyrene nanoparticles from plowing-induced surface wrinkling, Appl. Surf. Sci. Adv. 6 (2021) 100148
[2] J. J. Mazo et al., Plowing-induced structuring of compliant surfaces, Phys. Rev. Lett. 122 (2019) 256101
[3] P. Koczanowski et al., Stick-slip regimes accompanying atomic-scale plowing wear in molybdenum disulfide, in preparation

Prof. Mitjan Kalin (University of Ljubljana, Slovenia; Deputy President of the International Tribology Council, ITC)

Prof. Seong H. Kim (The Pennsylvania State University, USA)

Title:
Is Superlubricity an Intrinsic or Extrinsic Property of Hydrogenated Diamond-Like Carbon (HDLC)?
Abstract:
Hydrogenated diamond-like carbon (H-DLC) is produced as a coating through plasma-enhanced chemical vapor deposition. H-DLC is relatively hard and well-known for exhibiting superlubricity. This paper explores whether superlubricity is an intrinsic property of H-DLC. It argues that H-DLC is not intrinsically superlubricious; rather, it possesses an ideal structure that allows the interface region to transition to a superlubricious state under proper frictional shear conditions. Thus, its superlubricity is an extrinsic property. This argument is supported by comparing the frictional behaviors of three carbon allotropes-graphite, amorphous carbon, and diamond-and closely examining the run-in behavior and environmental sensitivity of H-DLC friction. While the superlubricious structure is generally believed to be graphitic, its exact nature remains elusive and warrants further study. Nevertheless, understanding how superlubricity is induced in H-DLC can inform engineering designs to achieve superlubricious behaviors in other carbon materials produced through different synthetic routes.

Prof. Dr. Evgeny A. Kolubaev (Institute of Strength Physics an Materials Science, Russian Academy of Sciences)

Title:
Mechanisms of Structure Formation in Friction Stir Welding and Processing
Abstract:
The main goal of the understanding of friction stir welding or processing (FSW/FSP) is a generalization of the structure formation patterns. The main factors determining the structure formation during FSW/FSP and the list of processes involved in the structure formation are defined. The localization of the zones of action of the main factors of structure formation and the structure formation processes realized during FSW/FSP is presented. Each of the factors influencing on the structure formation process in the mixing zone is determined by various interactions in the materials, as well as their initial structure during welding or processing. The main factors of structure formation include:
1. Adhesive-cohesive and deformation effect on the material, formation of additional high-angle boundaries and metal flow along the tool contour. Includes adhesive contact of the material with the tool, plastic deformation and formation of a fine-grained metal structure. Caused by the complex-stressed nature of tribological interaction during adhesive friction. Leads to the formation of a metal with a fine-grained structure capable of flow and transfer by an adhesive-cohesive or extrusive method, initiation of the material flow process and formation of the mixing zone structure.
2. Thermal effect on the material. Caused by adhesive friction and accompanying frictional heating of the material. Heating of the material in front of the tool is necessary for the FSW/FSP process, deformation, grinding of the material grains and superplastic extrusive flow of the metal along the contour of the tool. At the edges of the mixing zone, it causes the formation of a heat-affected zone. In the area behind the tool, it leads to grain growth, decomposition of supersaturated solid solutions and other processes depending on the type of material.
3. Structural and phase changes in materials during FSW/FSP of both complex alloys and composite materials with a metal matrix. Caused by heating, plastic deformation and mechanical mixing of dissimilar components with different mutual solubility in the adhesive contact zone of the tool and the material. Leads to the formation of supersaturated solid solutions or intermetallic phases in the solid state, as well as to the formation of eutectics of various compositions during contact melting.
4. Structural-phase, deformation, thermomechanical and tribological interactions of the material and the tool. Caused by processes occurring in contact between the material being processed and the tool metal. Depends on the process temperature, thermal resistance of the tool, mutual solubility of the material and the tool metal, etc. Leads to gradual wear of the tool and contamination of the mixing zone material with wear particles.
5. Initial structure of the material before processing, grain size and phase composition. Determines the resistance to tool movement during processing, the resulting structure of the mixing zone and the mechanical properties of its material. Manifests itself as a dependence of the grain size in the mixing zone on the initial structure and in the change in resistance to tool movement and/or torque on the motor shaft depending on the grain size and strength properties of the initial material. This factor determines structural and phase changes in materials and their interaction with the tool during friction stir welding and processing.

Prof. Alexander Korsunsky (Oxford University, UK)

Prof. Ken Nakano (Yokohama National University, Japan)

Title:
Viscoelastic toy model approach to stick-slip instability without friction laws
Abstract:
The relaxation oscillation in sliding systems is termed stick-slip since the two surfaces in contact seem to repeat the stick and slip states. However, several precise measurements have revealed that extremely slow slip occurs in even apparently stick states before every stick-to-slip transition, which is here termed the static friction paradox. This study theoretically investigates instabilities induced by solid viscoelasticity using a minimal toy model without friction laws: a two-degree-of-freedom sliding system consisting of a rigid probe and a Kelvin-Voigt viscoelastic foundation. Taking advantage of its simplicity, we discuss the mechanisms generating various system dynamics based on stability analysis and numerical simulation. As a result, surprisingly, even though the sliding system does not employ static friction, it exhibits intermittent oscillation resembling the typical stick-slip instability. It comprises two slip states for the contact between the probe and the viscoelastic foundation, slow-slip and fast-slip states, providing a purely mechanical explanation for the static friction paradox.

Prof. Dr. Salvador Pané i Vidal (Multi-Scale Robotics Lab, ETH Zurich, Switzerland)

Title:
Magnetic Small-Scale Robots for Biomedical Applications
Abstract:
We live in a world increasingly surrounded by robots such as robotic surgical systems, flying drones, autonomous planetary rovers, and robotic appliances. An emerging family of robotic systems are untethered magnetic micro- and nanorobots. These tiny vehicles can move in fluid environments by means of external magnetic fields. One of the ultimate goals of magnetic small-scale robotics is to develop machines that can deliver drugs, or realize other medical missions in confined spaces of the human body. Other applications include water remediation or “on-the-fly” chemistry.

The recent rapid developments in magnetic small-scale robotics are undeniably related to advances in material science, manufacturing and magnetic navigation. However, while many applications have been demonstrated, aspects such as complex locomotion and navigation, multifunctionality, biocompatibility and biodegradability need to be further investigated for the successful application of these devices in clinical settings.

In this talk, we will present recent advances and challenges for the realization of biomedical magnetic small-scale robots. We will specially focus on aspects that are key for their translation to the operating theatre

Prof. Valentin L. Popov (Technische Universität Berlin, Germany)

Title:
Is Griffith’ Energetic Criterion Applicable to Adhesive Viscoelastic Contacts?
Abstract:
For long time, it was believed that “first-principle” criteria like Griffith’ energetic criterion cannot be applied to viscoelastic contacts due to their dissipative nature. In the present paper, we argue that the energy balance principle is applicable in a pure and exact way to viscoelastic adhesive contacts too. This new approach sheds new light to many well-known phenomena in adhesion of viscoelastic bodies which had no clear theoretical explanation so far. In the presentation, various loading scenarios are considered and the influence of the rheology of elastomers on adhesion is discussed.

Prof. Dr. Josep Puigmartí-Luis (University of Barcelona, Spain)

Title:
Unlocking Materials Innovation with Microfluidic Technologies
Abstract:
Controlling and understanding the mechanisms that govern crystallization processes is crucial in contemporary materials science, particularly in the field of reticular solids, where significant challenges remain. In this seminar, I will demonstrate how microfluidic synthetic conditions can control the size and shape of various functional porous crystals, such as metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs). Specifically, I will show that microfluidic synthesis can produce the largest MOF single crystals with controlled nonequilibrium shapes reported to date, in contrast to the typical polyhedral microcrystals formed under bulk crystallization conditions. Additionally, I will illustrate how microfluidic technologies can address several challenges in the COF research area. For instance, I will demonstrate that a microfluidic device can enhance the processability of COFs, enabling the creation of macroscopic architectures composed solely of COFs with arbitrary shapes. This is particularly significant given that COFs are typically obtained as powders with limited solubility and no melting point, making conventional processing techniques like solution processing or melt-extrusion inapplicable, which also hinders their use in many potential applications. Moreover, I will also present how our group utilizes microrobotic platforms to apply MOFs and COFs in biotechnology and other advanced fields. These microrobotic systems enable precise 3D manipulation of MOFs and COFs, facilitating innovative applications such as targeted drug delivery, biosensing, and tissue engineering. By integrating microrobotics with our advanced synthesis techniques, we can create highly specialized and functional materials tailored for specific biomedical applications. This approach not only enhances the versatility and functionality of MOFs and COFs but also opens new avenues for their use in cutting-edge biotechnological solutions.

Prof. Alessandro Ruggiero (University of Salerno, Italy)

Title:
In-Silico Total Hip Replacement Wear: Novel Insightes and Perspectives in Synovial Lubrication Modelling of Rough Surfaces
Abstract:
The aim of this keynote was to describe the latest research results achieved at the Department of Industrial Engineering of the University of Salerno by the research group headed by the Author, in the framework of the computational (bio) tribological and biomechanical modelling of lubricated total hip replacements (THR), accounting for the possibility to consider the topography of the contact surfaces during the unsteady conditions. Main aim of the research was, in fact, to accurately predict the in-silico wear of artificial implants, modelling the complex tribological phenomena acting in the joints due to the synovial lubrication, considering unsteady loading of the joint and the real contact surfaces morphology observed during detailed optical analysis executed on retrieved systems. In this speech were underlined recent computational approaches obtained by merging multibody models, solving the inverse dynamics of musculoskeletal systems, and synovial mixed elasto-hydrodynamic lubrication models, also in presence of rough surfaces. The effectiveness of the proposed analysis consists in the possibility of examining many physical activities, characterized by cyclic kinematic and loading joint conditions like running, swimming and sport in general, in order to predict the implant duration overcoming excessive time and money consumption due to the experimental set-up and investigation, moreover taking into account the complexity of a mixed lubrication model adaptable to several synovial fluid lubrication properties and that considers the surfaces’ contact.

Prof. Sujeet K Sinha (Indian Institute of Technology Delhi, India)

Title:
Challenges in “Soft Matter Tribology” for Applications to Orthopaedic Implants
Abstract:
Nature’s lubrication is based on soft contact (cartilage-on-cartilage) and aqueous lubricant (synovial fluid) for the extremely low coefficient of friction and near-zero wear rate. This mechanism can be described as soft-elasto-hydrodynamic lubrication (soft-EHL) but the roles of the boundary lubricants and biphasic lubrication are immensely important. It is vital for the success of orthopaedic implants that the materials employed for artificial joints mimic the tribological functions of natural articular (hyaline) cartilage. The implant material must have softness of the cartilage with water absorbing biphasic property coupled with optimum strength, stiffness and fatigue properties. At the same time, a number of biocompatible boundary lubricants must be present in the material for continuous ultra-low coefficient of friction. A number of new polymeric composites have been tried, especially for the acetabular cup implant, with varying successes. In this talk, we will present the works conducted in biotribology of polymeric composites which have potential for future orthopaedic implant applications. The mechanisms of friction and wear will be explained in terms of soft contact mechanics, boundary lubrication and the soft-EHL and our current understanding of the design of new bio-composites will be discussed.

Prof. Hogyu Zhang (Tsinghua University, China)

Prof. Feng Zhou (Lanzhou Institute of Chemical Physics, China)