Fluctuating Hydrodynamics for Liquid Spreading over Heterogeneous Surfaces

Description: Understanding the spreading of liquids over heterogeneous solid surfaces is the key to numerous emerging technologies (e.g. capabilities to 3D print metallic structures), biological systems (retention of liquids by plant leaves) and even forensic science (analysis of blood splatters). The most remarkable feature of the wetting process is its strong multiscale nature, which ensures that nanoscale and microscale effects have a strong macroscopic influence on systems of interest. It is the fascinating multiscale nature of wetting that makes it such a theoretically challenging topic, as one must connect the microscale with the macroscale in a computationally tractable manner.

Aims: To develop a stochastic computational model capable of capturing the nanoscale fluctuations that drive macroscopic wetting flows over heterogeneous surfaces and validate this against molecular dynamics simulations, in collaboration with our partners in Mons.

Novelty & Methodology: Thermal fluctuations, which allow molecules to 'hop' over potential barriers in the solid, have long been considered as a potential driving mechanism for dynamic wetting flows. Owing to the small spatial and temporal scales on which this physics acts, molecular dynamics (MD) simulations are able to capture the local dynamics, and can be used as a benchmark. However, attempts to connect this mechanism to macroscopic dynamics, and thus experiments, have remained elusive due to the computational intractability of MD above the nanoscale. Furthermore, there is debate over the dominant forces for 'real' (i.e. heterogeneous) surfaces, where hopping over defects in the surface structure may be more important than the molecular jumps. These open problems can only be addressed by the development of a computational model based on the theory of fluctuating hydrodynamics, i.e. stochastic partial differential equation (SPDEs), which would give new, much-needed capabilities for understanding this system.

Alignment:

i) EPSRC Programme Grant 'Nano-Engineered Flow Technologies: Simulation for Design across Scale and Phase' (EP/N016602/1, PI: Lockerby)
ii) EPSRC CDT HetSys Core Training Objectives:
a) Interdisciplinarity - will be essential as the project cuts across physics (development of models), computational engineering and applied mathematics (e.g. SPDEs).
b) Robust Software Engineering - is required to develop both MD and SPDE computational models that will have lifetime and legacy beyond that of the project.
c) Uncertainty - will become important when attempting to quantify the influence of the random surface structure on macroscopic flow; a new direction of research in this field.

Collaborations:
i) The primary partner will be the Laboratory of Surface & Interfacial Physics in Mons (one of HetSys's partners) who have unrivalled expertise in the MD of wetting processes.
ii) This research should lead to collaborations with Professor Bruno Andreotti's group at Universite Paris-Diderot, where great experimental and theoretical progress has been achieved in the wetting of heterogeneous substrates, and the Centre for Smart Interfaces in Darmstadt.
iii) Any progress in understanding dynamic wetting could be translated to industrial partners who we have an existing relationship with, such as Bell Labs (HetSys partner), Schlumberger and Akzo Nobel.

Impact on Students. The primary impact will be on the 50+ PhD students trained by the Centre. They will be high-quality computational scientists who can develop and implement new methods for modelling complex systems in collaboration with scientists and end-users, who are comfortable working in interdisciplinary environments, have excellent communication skills and be well prepared for a wide range of future careers. The students will tackle and disseminate results from exciting PhD projects with strong potential for direct impact. Exemplar research themes we have identified together with our industrial and international partners: (i) design of electronic devices, (ii) catalysis across scales, (iii) high-performance alloys, (iv) direct drive laser fusion, (v) future medicine exploration, (vi) smart nanofluidic interfaces, (vii) composite materials with enhanced functionality, (viii) heterogeneity of underground systems.

Impact on Industry. Students trained by HetSys will make a significant impact on UK industry as they will be ideally prepared for R&D careers to help to address the skills shortage in science and engineering. They will be in high demand for their ability to (i) work across disciplines, (ii) perform calculations that come along with error estimates, and (iii) develop well-designed software that other researchers can readily use and modify which implements novel solutions to scientific problems. More generally, incorporating error bars into models to take account of incomplete data and insufficient models could lead to significantly enhanced adoption of materials modelling in industry, reducing trial and error, and costly/time-consuming R&D procedures. The global market for simulation software is expected to more than double from now to 2022 indicating a very strong absorptive capacity for graduates. Moreover, a recent European Materials Modelling Consortium report identified a typical eight-fold return on investment for materials modelling research, leading to cost savings of 12M Euros per industrial project.

Impact on Society. Scarcity of resources and high energy requirements of traditional materials processing techniques raise ever-increasing sustainability concerns. Limitations on jet engine fuel efficiency and the difficulties of designing materials for fusion power stations reflect the social and economic cost of our incomplete knowledge of how complex heterogeneous systems behave. High costs of laboratory investigations mean that theory must aid experiment to produce new knowledge and guidance. By training students who can develop the new methodology needed to model such issues, HetSys will support society's long term need for improved materials and processes.

There will also be a direct impact locally and regionally through engagement by HetSys in outreach projects. For example we will encourage CDT students to be involved with annual 'Inspire' residential courses at Warwick for Year 11 girls, which will show what STEM subjects are like at degree level. CDT students will present highlights from projects to secondary-school pupils during these courses and also visit local schools, particularly in areas currently under-represented in the student body, in coordination with relevant professional bodies.

Impact on collaboration. Our international partners have identified the same urgent challenges for computational modelling. We will build flourishing links with research institutes abroad with long term benefit on UK research via our links to computational science networks. Shared research projects will strengthen links between academic staff and industry R&D personnel and across disciplines. The work will also lead to accessible, robust and reusable software. The Centre will achieve cross-disciplinary academic impact on the physical and materials sciences, engineering, manufacturing and mathematics communities at Warwick and beyond, and on the generation of new ideas, insights and techniques.