Surface tension plays a role in many industrial problems and natural phenomena; many of these involve significant elastic effects of the basic materials such as the surrounding substrates and/or porous matrices, e.g. textiles, hair, and filters are just some of the soft materials that readily change shape (and possibly function) when wet or dried. Although static problems involving the interplay between surface tension (capillarity), elasticity, and possibly other forces are well studied, there are very few studies of the dynamics of elastocapillarity, which is the subject of this proposal. Because of the breadth of applications that involve elastocapillary dynamics, spanning material science, and the biological and environmental sciences, we believe that this research area will yield new insights and applications. The research in this proposal is focused on characterizing, both experimentally and theoretically, two common geometries - sheets and fibers - and two common dynamical situations - spontaneous wetting and drying. In addition, we will study the dynamics of wetting and drying of a model soft porous material consisting of a fibrous matrix. In this way, we will provide a systematic, yet broad characterization of the dynamics of elastocapillary flow problems. The research described in this proposal may yield new insights such as whether there is an optimum drop size for wetting a fibrous medium and how the drying rate of a fibrous medium is influenced by the softness (elasticity) of the fibers.
Broader impacts: In addition to publishing our findings in journals that span engineering, physics, and materials science, the PI will continue his multi-faceted outreach activities, including hosting visitors from different disciplines and educational institutions, engaging undergraduate students in research and giving talks at conferences, leading professional development activities for undergraduate, graduate, and postdoctoral colleagues, and participating in mentoring programs for young researchers from under-represented groups. Also, the PI has successfully incorporated research themes into "holiday" lectures for children and parents that he has given over the past 9 years and which he will continue to develop and present. Overall, the research will have a direct impact on our understanding of wetting and drying of common soft fibrous materials, which arises in a wide variety of industrial and natural systems.
Liquids are observed commonly to wet solids. We have studied such wetting phenomena on fibrous materials, for which a pair of fibers is a model system. Although it might be expected, based on experience with other ssystems, that the wetting characteristics are controlled by the surface chemistry, here the way in which the drop contacts and covers the fibers also depends on the spacing between the fibers, which is a geometric feature (see figure 1, left). We studied many aspects of this system and also then studied how such systems dry. For example, we observed, and then explained with quantitative models, how the rate of evaporation of the liquid is significantly impacted by the spacing between the fibers (see figure 1, right). In this way, we now understand ways that mechanical effects can impact the evaporation from fibrous materials. In addition, we tried to extend this understanding to more realistic systems and have studied the wetting of liquids on crossed fibers (see figure 2). Finally, we have used this understanding to study several problems where deformation of the substrate is coupled to the shape of the liquid, which is the theme of studies referred to as elastocapillarity. Our work is an example of physical problems that lie at the intersection of fluid mechanics and elasticity and so forms part of the interdisciplinary topic of fluid-structure interactions. Throughout our work we have engaged young people in our studies. We have worked with undergraduate students, graduate students and postdocs, and hosted students from other universities as well as overseas. We have mentored the young people and they have had the chance to see research progress from a new idea, to new results, to a journal publication and conference presentation. In addition, we have communicated ideas of science and engineering to the public via various activities at the university as well as public lectures.