Superhydrophobicity, bubble stability, and heterogeneous nucleation
Wetting on rough surfaces and bubble nucleation at solid interface are two important (theoretical and applied) research fields that, at a first glance, appear to be far from each other and that, with the exception of very few research groups, are tackled by different scientific communities. Concerning wetting, in the last couple of decades, the increasing interest for miniaturised fluidic devices has been accompanied by the need of a deeper understanding of how the chemistry and the geometry of the solid substrate affect the wetting of rough surfaces. For instance it is well known that a rough surface with an hydrophobic coating promotes the entrapment of air or vapor within asperities, resulting in remarkable macroscopic properties, such as high contact angle and self-cleaning, that are collectively known as superhydrophobicity. The reason for the growing interest is the availability of micro- and nanofabrication techniques that have allowed to realize synthetic air-capturing superhydrophobic surfaces by precisely controlling the chemistry and topography of surfaces. However in spite of the recent technological advancements, the stability of the entrained bubbles and the mechanism of wetting remains largely elusive, making it difficult to predict how long a superhydrophobic state will last and to design effectively synthetic surfaces.
Concerning vapour (or gas) bubble nucleation at solid interfaces, commonly indicated as heterogeneous nucleation or heterogeneous cavitation, the topic have been traditionally studied in the field of hydraulic and marine engineering where cavitation is a much-feared phenomenon that leads to the rapid deterioration of fluid machinery and hydraulic structures. The abrupt collapse of cavitation bubbles is indeed accompanied by intense increase of the local pressure and temperature. Recently cavitation was studied also in other fields such as material science, where the extreme conditions in the hot spots due to bubble collapse have been favorably exploited in the synthesis of nanostructured materials and in medicine, where cavitation induced by ultrasounds shows promise as a noninvasive tool capable of in situ destruction of damaged tissues and occlusions. It is well established that bubble nucleates preferentially on surface defects (especially when combined with hydrophobic coatings) and that gas nuclei trapped at the solid interface act as catalysts to accelerate nucleation events. Hence, also heterogeneous nucleation is a phenomena where the interplay of geometrical and chemical features of the solid surface plays a crucial role.
This workshop aims at bringing together researchers working on both wetting and cavitation on rough surfaces to share ideas and different approaches between the two communities. Experimental, theoretical, and numerical approaches will be presented by top-level researchers in the two fields. Round tables about specific topic and tools (e.g. the use of rare events techniques and innovative continuum description of nucleation and wetting) are envisaged. The subject is potentially considered of significant relevance to CECAM given the need to develop advanced numerical tools to tackle phenomena occurring typically on an extended range of spatial and temporal scales. Indeed specialized techniques borrowed from cutting edge statistical mechanics may prove ground breaking tools in this field while, at the same time, the macroscopic consequences of wetting and heterogeneous nucleation call for innovative continuum modeling. Considerable attention is paid in the workshop to extract at the end of each meeting day the current direction of experiments, applications and their interplay with modeling issues. The final purpose of the workshop is planning a road map, designing the research directions, and fostering multidisciplinary interaction aimed at developing a new generation of wetting and nucleation models.
The theoretical description of the effect of roughness on surface hydrophobicity dates back to the ‘30s and ‘40s. In their seminal work, Wenzel , Cassie and Baxter , and Pease  have characterized the states of a droplet deposited on a non-smooth surface. A droplet can completely wet the surface corrugations (Wenzel state) or float over air/vapor bubbles filling them (Cassie-Baxter state). The contact angle of a droplet is higher in the Cassie-Baxter than in the Wenzel state, and both have a contact angle higher than the same liquid on the corresponding smooth surface. Thus, the Cassie-Baxter state is called superhydrophobic.
Recent technological progress made it possible to create surfaces with textures of suitable size and shape that can be used for a wide range of applications: self cleaning materials, drag reduction , open nanofluidic channels for MEMS (Microelectromechanical systems)  and many more . These applications are associated to the ability of these surfaces to maintain a liquid (typically water but also other liquids such as oils) in the Cassie-Baxter state. The present generation of superhydrophobic surfaces is inspired from natural surfaces, such as the lotus leaves, and designed according to empirical relations derived from the Cassie-Baxter and Wenzel models. However, an ever growing number of experimental results show that the condition of existence of the superhydrophobic state cannot be described by the aforementioned empirical relations, and that the geometry of the textures has a significant influence on it . These results, complemented by the observation that naturally selected superhydrophobic or superomniphobic surfaces present a wide variety of surface configurations [8,9], suggest that much remains to be investigated on the relation between surface chemistry and topology, and the properties (hydrophobicity and slippage) and range of (meta)stability of the Cassie-Baxter state. Another, related, subject we intend to discuss in this workshop is the heterogeneous nucleation of gas and vapor bubbles in liquids on surfaces, especially in presence of micro and nano textures. Bubble nucleation (cavitation) and collapse can have positive and negative technological applications. In the field of hydraulic and marine engineering cavitation is a much-feared phenomenon that leads to the rapid deterioration of fluid machinery and hydraulic structures [10,11]. The sudden and intense increase of pressure and temperature accompanying the collapse of the bubbles can be favorably exploited in the synthesis of biomaterials [12,13] and nanostructured materials [14-16]. In medicine, cavitation induced by ultrasounds shows is promisingly used in therapy, imaging, and drug delivery [17-21].
The theoretical description of bubble nucleation is, historically, divided in two cases: nucleation in (i) presence and (ii) absence of dissolved gas. The theory of vapor nucleation is due to Volmer and dates back to late 30s , and, for the homogeneous case, coincides with the Classical Nucleation Theory. For the case with dissolved gas, perhaps the most successful model is the crevice model in the formulation of Atchley and Prosperetti , which establishes the condition for regression or growth of a gas bubble. A recent work  has shown that both cases can be treated within the same framework. Indeed, surface textures can significantly enhance cavitation. This enhancement is related to the existence of Cassie-Baxter and Wenzel states and to the rate of transition between these states. Thus, investigation of the stability of the Cassie-Baxter/Wenzel states and the related phase transitions is a key element for understanding cavitation in novel nanofluidic devices and smart surfaces.
Carlo Massimo Casciola (Università degli Studi di Roma La Sapienza) - Organiser
Simone Meloni (University of Ferrara) - Organiser