Capillary interactions on soft and textured surfaces

Capillary interactions are crucial for many aspects of our lives, from the shapes acquired by water drops on grass and leaves to scientific applications such as lithography and liquidmediated adhesion. In this thesis, we explore three systems where capillary interactions play a key role: Firstly, an evaporating drop on a soft micropillared substrate. Secondly, a drop being squeezed between two superhydrophobic surfaces, and thirdly, a glass sphere in contact with a thin viscous oil film.
In chapters 2 and 3 of this thesis, we deal with structured solids that display waterrepellent properties. In chapter 2, we start by investigating a soft microstructured surface:
a polydimethylsiloxane (PDMS) micropillar array. Our experiments show how capillary forces can cause slender pillars on the array to deform and collapse onto one another. This
had previously been studied. However, the kinematics of the process had remained unexplored due to the difficulty in imaging the individual menisci between the pillars at the time of the collapse. We visualized this process through confocal microscopy and found that a key ingredient in capillary self-assembly and collapse is the local contact angle of the meniscus between the pillars. We do this by imaging the impalement process of a water drop on the PDMS micropillar array in two scenarios: a classic superhydrophobic state (with air between the pillars) and an infused case (with FC-40 between the pillars). Our observations lead us to develop a simple model, with three consecutive 2D pillars, that includes the local contact angle, stiffness of the pillars, and interfacial tension to predict the onset of the structural collapse. The model accurately reproduces the onset of bundling seen in our experiments.
Chapter 3 moves on to rigid superhydrophobic micropillared surfaces. In that chapter, we focus on compression tests of a drop between two superhydrophobic surfaces. We
image the compression of a water drop from the top while simultaneously measuring the compression force on one of the surfaces. The Cassie–to–Wenzel transition is evident in our experiments from both types of measurements, and with this, we calculate the experimental impalement pressure of our superhydrophobic surfaces. We then discuss our experimental results with the current impalement pressure estimations, which show that our experimental values lie below the current predictions. This emphasizes the difficulty in accurately assessing the pressure threshold for the Cassie–to–Wenzel transition with the current models.
In Chapter 4, we move on to smooth solids and study the formation and evolution of a liquid bridge between a sphere and a flat solid covered by a thin viscous film. In this chapter, we investigate the radial growth of the liquid bridge and its correlation with the dynamics present in the thin viscous film. Our experiments use a combination of imaging techniques; bright–field and digital holographic microscopy. This allows us to describe in detail the rich dynamics at play when a liquid bridge is forming from a thin viscous layer. We reveal that the liquid bridge growth displays two distinct dynamic regimes: an early fast-growing regime, and a late slow-growing regime. This is of particular importance as a dynamic liquid bridge gives rise to time-dependent capillary adhesion forces.

Original promotion date: January 14, 2022 (COVID-19)