The dynamics of hygrotactic fronts - from fundamentals of soft-matter systems to smart materials engineering applications

Tactic behaviour is understood as the ability of a system to sense and respond to external stimuli. This tactic behaviour is present in many complex systems, many of which can be found in nature. Tactic systems are able to sense external stimuli (for example, chemical or temperature gradients) and convert this sensory information into large-scale self-propulsion. This PhD thesis aims to explore a transformative concept for tactic behaviour in soft-matter physics and smart-materials engineering called hygrotaxis, defined as the motion of a droplet on a solid surface caused by the presence of a humidity gradient. The interactions present in this effect give rise to very small forces. Hence, experiments are carried out on Slippery Omniphobic Covalently Attached Liquid-like (SOCAL) surfaces, a type of ultra-smooth surface which exhibits remarkably low contact angle hysteresis (< 3◦). Base studies on the evaporation dynamics of water droplets on ultra-smooth surfaces show that SOCAL was successfully grafted into glass substrates. Studies of the contact angle hysteresis on these surfaces shows there are different friction regimes that make measurement of contact angle hysteresis challenging. Hence, a novel method of characterisation of ultra-smooth surfaces is developed to use the relaxation of interfaces to measure contact angle hysteresis on SOCAL surfaces. The relaxation times obtained in these studies allow for the analysis of the physical mechanisms guiding the motion of the droplet, which depends on the kinematics of the contact line rather than the hydrodynamics. Lattice Boltzmann simulations are implemented to study the effect of humidity gradients on liquid droplets inside a square channel. Base simulations show that the Lattice Boltzmann algorithm is accurate in modelling the dynamics as predicted by the Cahn-Hilliard equation. Gradients inside the channel are generated by introducing a chemical potential difference in the ambient phase. These simulations show that chemical potential gradients are able to affect the liquid-gas interface, generating surface flows that guide the droplet towards more humid environments. Based on the findings in this thesis, two experimental approaches are developed to study hygrotaxis. The first approach aims to generate a gradient in relative humidity by placing two droplets of significantly different sizes in close proximity (∼ 0.1 mm) in an ambient relative humidity of 10%. The effect observed is a consistent displacement of the centre of mass of the small droplet towards the larger one. Comparisons between experiments and simulation show an overall agreement in the behaviour of the droplet in such configuration. The second approach consists of separating the exposed surface of a small amount water into two different interfaces. This is done by developing a method to coat and characterise the inside of glass capillary tubes with the SOCAL coating. Comparisons between coated and uncoated samples show that SOCAL has been successfully applied, exhibiting contact angle hysteresis measurements of 2.9◦ ±1.9◦. Although pinning events are prevalent in SOCAL coated samples, evaporation of 2μL liquid slugs indicate the possible existence of a pressure driven effect which drives the contact line to recede from the humid side rather than the dry one. Even though improvements in the experiments still have to be implemented, these results are motivating and serve a proof of concept for future studies. The methods and the results presented in this work improve our understanding of the interaction of water droplets, ultra-smooth surfaces and relative humidity gradients