Friction, under a broad definition including the interaction between solid-solid and solid-fluid interfaces with relative motion, is a cause of reduced efficiency of motion on many scales, inducing wear, vibration, drag and other phenomena associated with parasitic energy consumption. The multiscale nature of the phenomenon, arising from complex system dynamics that are both driven by and coupled to frictional forces, have limited the engineering community's ability to describe the process in a deterministic manner.
Improved understanding and modeling of the multiple origins of friction form a vital component of technologies aimed at reducing the environmental impact of human development and transportation. The implications for solid-solid interfaces include reduced wear, noise, vibration, etc. and increased transmission efficiency in many applications. At interfaces between fluid flow and solid materials this enhanced understanding holds potential for advances across disciplines, ranging from turbulent skin friction reduction for increased vehicle efficiency, improved models and control of pollutant dispersion by atmospheric winds in urban "rough" environments to advanced understanding of environmental physics and climate phenomena such as ice-field development and glacier melting. The need for significant progress challenges the engineering community both to find interdisciplinary approaches that integrate elements of fundamental solid and fluid mechanics and to develop active control techniques to shape the coupling between friction at the interface and the overall system response in optimal ways.
We have assembled a group composed of experts in theory, experiment and simulation from both the fluid mechanics and solid mechanics communities, including leaders in the fields of microfluidics and slip, turbulent boundary layers, geophysical and environmental flows, response and modeling of multiscale dynamical systems, lubrication flows, tribology and contact mechanics. A degree of "cross-fertilization" of methodology and understanding between communities is sought. It is hoped that the exchange of ideas arising during and after this workshop will lead to advances in both fields, with the potential for publications detailing the similarities, differences and challenges encapsulated in the problem of friction at various interfaces.
The objective of the workshop can be captured in identifying the answers to the following questions:
Solid and fluid friction: How are these problems the same? How are they different? What can the communities learn from each other?
Discussion points will include:
- The multi-scale nature of frictional interactions and associated system response.
- Solid-solid interfaces, solid-fluid interfaces and lubrication flows.
- How can the "essence" of friction and appropriate techniques to understand its effect on the interacting systems be distilled from the various participating disciplines?
- System response mechanisms and measurement of techniques to identify sub-processes.
- Characterization of surface roughness and asperities with a view to predicting system response.
A grand challenge lies in the formulation of unified analysis techniques addressing friction through the momentum/energy transfer and thermal processes leading to the diversion of useful kinetic energy to parasitic processes on atomic to continuum scales, irrespective of the media on either side of the interface.
The otherwise disparate processes by which momentum is introduced, distributed and removed from a fluid medium adjacent to and in relative motion with a rough boundary and by which strain energy is introduced, distributed and removed from two solids in sliding contact at a rough interface, have one commonality. A full description of both processes requires their resolutions as sub-processes, distinguished by the space/time locations and the space/time scales for both the input (momentum/energy extraction via the driving forces) and output (dissipation) fields. Two component sub-processes are intuitive. One is the transport of momentum captured in a variation at the interface that is limited to a narrow-band of space-/time-scales across space-/time-locations. The second is the local (in space/time) transfer of energy within (an intra-scale process) and across (an inter-scale process) limited space/time-scale bands.
On the abstract level of a mathematical "framework" on which to construct an analytical model, the described commonality is paramount. Thus, accomplishing a model either in space-/time-location space or in space-/time-scale space, folds together sub-processes, which retain their separateness modeled in combined space-/time-location and/or space-/time-scale phase-space. The hypothesis to be considered, discussed and validated or confounded is as follows:
A new generation of models for the mechanical processes where either momentum is transferred in a fluid near a rough solid boundary or where strain energy is distributed in two solids in sliding contact at a rough interface can be constructed using a common mathematical framework formulated in a phase-space.