Näkökulmia kapillaarisiltoihin: näennäis-kaksiulotteisia pisaroita sekä elävien solujen muodostamia siltoja

Liquid-solid and liquid-liquid interactions are central to fundamental research, impacting fields from physics to biology. Liquids can range from simple, like water, to complex, including living systems. The interactions often involve coupled capillary effects that are challenging to quantify separately. Confining the system offers a way to reduce complexity, albeit more readily accomplished in theoretical models than in experiments. This thesis presents perspectives on capillary bridges, including both confined systems of liquid droplets and living tissues, to explore capillary interactions and their implications in three-dimensional spaces. Publication I focused on liquid-solid interactions on superhydrophobic surfaces, where minimal contact between the droplet and solid is provided by trapped air, called a plastron, in between the droplet and solid surface with microscopic pillars. By measuring friction forces and plastron thickness simultaneously, a friction component originating from the plastron was identified. This effect resulted in significant dissipations at larger droplet velocities, reducing the performance of the surfaces. Therefore, the results from this work facilitate improved design of water-repellent coatings. Publication II transitions three-dimensional (3D) droplets into a quasi-two-dimensional (Q2D) framework. Gravity effects were retained despite the geometric transition, resulting in droplet sizes and shapes determined by capillary length, and falling and sliding droplets similarly as to 3D. Publication III focused on liquid-liquid interactions utilizing the Q2D geometry to investigate liquidliquid interactions. The confined geometry provided optimal conditions to investigate vapor and thin film mediated mass transfer that resulted in complex Marangoni effects. Both long- and short-range vapor effects were identified, influencing droplet motion and thin film formation. The results from Publication II and III together demonstrated effective Q2D droplet analogies, from static to dynamic ones, valuable for studying various interactions. In Publication IV, confined geometry was applied to living tissues, examining complex liquid-solid interactions by creating living capillary bridges that resemble inert liquid bridges. Dynamics driven by cellular activities caused transitions from quasi-static to unstable bridge states, leading to selforganized rupturing. This transition was influenced by the competition between cell growth and cell flow, suggesting a mechanical interplay affecting tissue integrity in confinement. This thesis presents perspectives on capillary bridges and demonstrates that they are valuable techniques for studying various liquid-solid and liquid-liquid interactions, particularly in scenarios involving complex liquids, interactions, and even living matter. The key advantages include welldefined and controlled interfaces and optical access, facilitating easier study of 3D problems and the possibility of discovering new effects due to boundary conditions from the confinement.