Gels with tunable mechanical properties and rheological behavior are at the core of new material technologies. All soft matter, in fact, from proteins to colloids or polymers, easily self-assembles into these weakly elastic solids [1-5]. The nano-scale size of their building blocks make their structure quite sensitive to thermal fluctuations, with a rich relaxation dynamics associated to spontaneous and thermally activated processes [6-10]. In addition to affecting the time evolution, or aging, of the material properties at rest, those dynamical processes also interplay with an applied deformation and are crucial for their mechanical response. The result is a complex rheological behavior, with diverse, unexpected and strongly non-linear features (shear localization, stiffening, creep and fracture…) [11-16].
Disentangling the impact of the structure, from dynamics and rheological response is hard, due to the complexity of the gel structures and of the microscopic dynamics at play. Theoretical and computational work in past years has been mainly focused on the self-assembly and structure formation associated to gelation of colloidal suspensions or protein solutions and on their implications for the microscopic relaxation dynamics [17-20]. Nevertheless the connections with the mechanical properties and the non-linear rheological response of the solid gel networks remain poorly understood, significantly hindering new progress for material technologies.
In the last years novel technologies have allowed experimentalists to combine rheology with imaging, ultrasound velocimetry or spectroscopy [21-25]. Such approaches provide now unique opportunities to fill the gap between the macroscopic rheological behavior of the material and its micro- and even nano-scale structure/dynamics. Recent computational efforts have been devoted to address non-linear response, role of hydrodynamic interactions, and structural/mechanical heterogeneity [26-31]. Finally, new developments in rationalizing non-linear response, yielding and relaxation in amorphous soft materials offer new mesoscale theoretical frameworks that can help bridging the microscopic physics to the macroscopic, engineering properties of gel materials [32-36]. The insight gained can help address the issues of how mechanical stability and rigidity emerges, how elasticity may affect the microscopic dynamical processes and how structural heterogeneities impact the non-linear rheological response [37-46]. We propose therefore a workshop that brings together scientists working on these recent developments with theory, simulations and experiments, to analyze and discuss the emerging physical understanding. While consolidating and analysing critically the novel findings, we will identify challenges and open questions for material technologies and applications .
The idea of this conference is to bring together international experts in the field of the dynamics of gel networks, to discuss recent progress and insight from computational, theoretical and experimental approaches. This field is very young especially concerning modeling aspects, due to the computational challenges related to the complex molecular structures which are crucial, in many cases, for relaxation dynamics and mechanical responses on large scales. In the recent years, however, the computer power and new parallel coding techniques like GPGPU or large scale MPI programming techniques has allowed for significant progress in accessing reasonable time and length scales to address questions that were difficult to tackle before. Also on the experimental side, new approaches combining different techniques (e.g. various types of spectroscopy and rheology) allow for an investigation on a more mesoscopic scale of the complex dynamics, being able to probe scales that can be modeled within, for example, MD simulations. In addition, recent theretical developments now address specifically the non-linear response of soft materials, providing new concepts and ideas to be tested in experiments and simulations. There seems to be a unique opportunity, at this point, in combining such efforts to develop a new understanding of gels, closely related to their technological applications. This is why we think it is extremely timely and important to bring together the different relevant communities with the help of CECAM. The specific topics and questions that we would like to focus on are the following:
Aging and Relaxation:
Time evolution of structure and material properties; stretched and compressed exponential relaxation; elastic effects.
Structural Heterogeneities, Coarsening and Restructuring: Role of coarsening dynamics in phase separating suspensions and mixtures; changes of the structure upon forcing or relaxation; interplay between structural and stress heterogeneities.
Gel Rheology at Small and Large Amplitude Deformations: specificity of the rheology of networks with respect to dense systems; connection between the linear and non-linear response of gels and their microstructure; fatigue and rheological hysteresis in terms of damage accumulation in soft amorphous solids.
Rigidity Percolation, Soft Modes and Stressed States:
onset of mechanical stability and rigidity percolation; role of frozen-in stresses in the gel structures; marginality and soft modes in disordered networks related to frozen in stresses; specific caracter of non-affine rearrangement.
Creep and Precursors to Failure: critical slowing down of dynamics as stress decreases and related scaling laws; microstructural origins of the creep; possibility to define precursors; materials durability.
Design and Control of Mechanical Instabilities: hardening, stress and strain localization; controlling structural damage; coupling between the emergence of strongly non-linear response and specifically designed structural/micromechanical features; role of walls and confinement under deformation.
The ambition of this workshop is to initiate and intensify the discussions between different communities working on mechanical properties of network forming systems on different scales, bridging the gap between experimental observations, simulations and coarse grained theoretical descriptions. We intend to bring together a group of leading experts in the field of computational modeling of gels (molecular dynamics and mesoscopic scale descriptions) with leading experts in the experimental community. We aim at: (i) identifying the most important open questions in the field regarding the link between structure and mechanical response, (ii) analyzing the computational approaches needed to address these questions and (iii) building a consistent multi-scale approach of the rheology of gels which combines the micro, meso and macro scales.