Wetting of Elastic Fibres: A Novel Immersed Boundary-Lattice Spring-Lattice Boltzmann Simulation Approach

Fibrous structures are versatile materials. They are abundant in nature, as observed in feathers, hairs, spider webs and adhesive pads of insects. They are also widely exploited in engineered systems, from the familiar examples of papers and textiles to high-precision micro- and nano-technologies.

For many fibrous materials, their interaction with liquids is of paramount importance. Due to the small size of the fibres, capillary action (as observed e.g. in the drawing up of liquids in plants) often plays the dominant role. Furthermore, the action of capillarity deforms the fibres, which results in an opposing force due to elasticity. This competition between elasticity and capillarity - elastocapillarity - finds its relevance in a wide range of applications, including liquid penetration in wipes and nappies, and the clumping of hairs in the tarsi of insects and of barbules in marine bird feathers.

Despite the numerous industrial applications and common occurrence in nature, our understanding of elastocapillary response of wet fibres is still very limited. Recent experiments suggest that pattern formation in nanostructures can be manipulated by elastocapillary dynamics; the capture of drops and their splashing, of relevance to the application of pesticides or herbicides among others, depend on the fibre geometry and flexibility. These are just a few of many elastocapillary phenomena ripe for investigations, especially using computer simulations, since the intricate interplay between fibre geometry, elasticity and capillarity quickly makes analytical treatments intractable. Unfortunately, simulation methods that can capture solid deformation, flow of liquids, and capillary forces are currently not available.

Thus, it is my aim in this project is to deliver a numerical platform able to tackle such a challenge. This project is rooted in my recent research advances in simulation techniques for wetting phenomena. As a proof of principle, to demonstrate that the novel method can capture wetting dynamics on soft materials, I will examine the spreading of small droplets on two elastic fibres oriented at various angles with respect to each other, and I will study the removal of these droplets under the action of a body force such as gravity. These are paradigmatic examples for understanding the arrangement and cleaning properties of natural and synthetic wet fibre assemblies.

My new simulations will be validated against experimental data provided by Procter and Gamble, where wetting of elastic fibres is relevant for many of their products, ranging from adsorbent materials (including nappies and wipes) to personal hygiene products (e.g. shampoos and laundry detergents).

If successful, my novel approach will open an unprecedented route to model static and dynamic elastocapillary phenomena embedded in complex geometries. As such, it will advance our understanding of elastocapillarity, and help channel fundamental scientific insights into design principles for practical applications.

1. Knowledge and Technique
This project will produce a novel simulation method for elastocapillary phenomena, as well as new and fundamental insights into wetting dynamics on flexible fibres. In particular, the ability to simultaneously account for solid mechanics, fluid dynamics and interfacial forces is powerful and systematic future investigations will open an unprecedented modelling route to translate fundamental insight on elastocapillarity into design principles for novel and smart materials. This project will strengthen the priority themes of "Soft Matter and Biophysics" and "Complex Fluids and Rheology" as defined by EPSRC, and it aligns closely with EPSRC's "Frontier Manufacturing" research challenge under the "Manufacturing the Future" theme.

2. Economic Impact
A key priority for the UK is in high-value manufacturing. The development of a robust simulation method for elastocapillary phenomena will help us move away from empirical design process (trial and error) to "frontier manufacturing" where scientific principles are used from the start.

Indeed, elastocapillarity is a key technological driver for a wide range of industrial sectors. Here I have collaborated with the company Procter and Gamble (P&G), focussing in particular on the wetting of elastic fibres. Elastic fibres are an excellent model system for fabrics and hairs among others, and as such, this project is relevant for consumer products such as nappies and shampoos. For these two products alone, the global industrial market is forecasted at approximately $60 billion and $25 billion respectively by 2020. Wet fibres are also relevant for imbibition in filtering devices, wet lithography process in micro-electromechanical systems, and the capture of pesticides in agriculture, to list a few examples.

The link with P&G provides an excellent opportunity to deliver impact of relevance to an industrial context. The PDRA and I will sign a confidential disclosure agreement with P&G, and IP rights will be arranged with the help of Durham University Business & Innovation Services. P&G's support to this proposal includes staff time, access to their experimental data, and travel costs for two research visits.

Additionally, I will engage other industrial partners in non-competing sectors with interests on elastocapillarity through relevant academic-industrial networking events, such as the "UK Soft Matter Showcase and Industry Day" organised by the Soft Matter and Functional Interfaces Centre for Doctoral Training based in Durham and the "European Study Group with industry" meeting. I will also invite industrial representatives to a workshop on wetting phenomena I will organise as part of my "Pathways to Impact" activities.

3. Training
The development of subject specific and soft skills of the PDRA will contribute to the highly skilled workforce in the UK. The PDRA will gain expertise in cutting-edge computational modelling techniques and in the field of wetting phenomena, through close collaboration with me. The PDRA will take advantage of Durham University's Research Training Programme, which won a Times Higher Education award for Outstanding Support for Early Career Researchers. Furthermore, the industrial link with Procter and Gamble provides career development opportunity for the PDRA beyond the standard academic settings,

As PI, I will benefit from the experience in leading this EPSRC First Grant project. I will gain leadership and management skills, and I will broaden my research portfolio. The delivery of a numerical platform capable of combining solid elasticity, fluid mechanics, interfacial and wetting phenomena will distinguish my group, and allow me to strengthen and build links with leading experimentalists and industrial partners in the area of elastocapillarity. All these will help me achieve my ambition: to build a world leading research group in the field of wetting phenomena, and more generally, soft condensed matter.