Session M01: Drops: Interaction with Elastic Surfaces, Particles and Fibers

Curvature driven propulsion of floating films: Part 1

Small floating objects often clump together, due to the long-range capillary interactions between their boundary menisci. Previous studies of the “Cheerios effect” [1] have focused on rigid solids, but much less is known about analogous behaviors for floating objects that are easily deformed by surface tension. Here the behaviors may be much more subtle, and can be strongly influenced by geometric incompatibilities between the solid and the fluid. We focus on a model system where a thin polymer film (~100 nm) is confined to the curved water interface of an overfilled petri dish. We measure the trajectories of planar films and curved shells released from various starting positions. By altering the geometry and thickness of the films and the viscosity of the fluid, we build an empirical description of this system, in regimes where the drag is primarily inertial or viscous. In all cases, the spontaneous translation is directed towards a region where the curvature of the meniscus is similar to the rest curvature of the film. In the following talk, we describe simulations that we use to measure the system energy. (This is part 1 of a 2-talk series.)

[1] Vella & Mahadevan, Am. J. Phys. 73, 814 (2005).

Support from NSF-DMR-CAREER-1654102 is gratefully acknowledged.   

Curvature driven propulsion of floating films: Part 2.

Thin films readily buckle and wrinkle to form a variety of three-dimensional shapes of general curvature. Motivated by the self-propulsion of floating films on a curved water bath, we perform Surface Evolver simulations to probe the energy of placing a thin, easily-wrinkled film on a curved liquid interface. Our simulations use a coarse-grained approach where the film is modeled using a triangulated surface with edges that can compress but not stretch [1]. The system energy is comprised of the gravitational energy of the liquid, and the surface energy of the liquid that is not covered by the film. By measuring the energy as we vary the location of the film, we show how the film experiences a force directed away from a curved meniscus towards a central flat region of the liquid interface, consistent with our experiments using ultrathin polymer films. Although these behaviors are perhaps reminiscent of the capillary migration of solid particles [2], the phenomenon studied here is intrinsic to elastic films that are easily wrinkled yet hard to stretch. (This is part 2 of a 2-talk series.)

[1] Paulsen et al., Nature Materials 14, 1206 (2015).

[2] Cavallaro et al., PNAS 108, 52 (2011).   

Dropwise Condensation and Droplet Shedding on Actuated Thermally Enhanced Hydrophobic Tubes

Recent advances in soft, stretchable, thermally enhanced (through the addition of liquid metals) silicone tubing offer the potential to use these stretchable tubes in place of conventional copper pipe for applications such as dehumidification. Copper is a common material choice for dehumidifier evaporator tubing owing to its ubiquity and its high thermal conductivity, but it has several thermal downsides. Specifically, copper tubes remain static and typically rely on gravity alone to remove water droplets when they reach a sufficient mass. Additionally, copper’s naturally hydrophilic surface promotes filmwise condensation, which is substantially less effective than dropwise condensation. In contrast to copper, thermally enhanced soft stretchable tubes have naturally hydrophobic surfaces that promote the more efficient dropwise condensation mode and a soft surface that offers higher nucleation density. However, soft surfaces also increase droplet pinning, which inhibits their departure. In this presentation, we experimentally explore the effects of stretching and retraction of soft tubing internally cooled with water on droplet condensation dynamics on its exterior surface. We discuss the results in terms of device and droplet dynamic timescales.   

Coating a fiber with an elastomeric shell

Coating cylindrical fibers with a geometrically uniform shell is of great importance in many different industrial and biological applications. As a viscous liquid coats the outer surface of a fiber, capillary effects lead to the Rayleigh-Plateau instability and the growth of initial perturbations in the coating profile. This leads to undesirable beads that form periodically along the fiber and consequently a non-uniform final coating. It is known that bead formation can be avoided, for vertically-held fibers, through a gravity-driven primary flow in the annular film. We investigate this free-surface flow phenomenon and study its applications under different flow conditions. We coat cylindrical fibers with viscous polymer solutions that gradually go through a sol-gel transition and transform into permanent elastic shells around the fiber. The kinetics of the gelation process is coupled with the spatiotemporal evolution of the interfacial disturbances in the coating layer and, together, set the final profile of these elastomeric cylindrical shells. We analyze the final profile of these elastic shells and discuss our results in view of a simple analysis based on the nonlinear saturation of the Rayleigh-Plateau instability.   

Dynamic elasto-capillarity in ternary immiscible flows

Elasto-capillarity involves the deformation of an elastic solid due to the capillary forces at a fluid-fluid interface. In recent years, multiple intriguing experiments involving elasto-capillary phenomena have been reported, including spontaneous migration of droplets on deformable surfaces with stiffness gradients (durotaxis) or strain gradients (tensotaxis), self-wrapping of liquid droplets when placed in contact with elastic membranes and wrinkling patterns arising due to the placement of liquid drops on ultra-thin sheets with low bending stiffness. However, the interaction of highly deformable solids with compound droplets has received less attention. Here, we develop a high-fidelity computational method that enables simulation of three-component flows with a nonlinear elastic solid. This talk will present multiple examples illustrating the complexity and richness of this system.   

Damping by dampening

In this study, we explore the vibration damping characteristics of singular liquid drops of varying viscosity and surface tension resting on a millimetric cantilever. Cantilevers are displaced 0.6 mm at their free end, 6% their length, and allowed to vibrate freely. Such ring-down vibration causes drops to deform, or slosh, which dissipates kinetic energy via viscous dissipation within the drop and through contact line friction. Damping by drop sloshing is dependent on viscosity, surface tension, drop size, and drop location. A solid weight with the same mass as experimental drops is used to compare against the damping imposed by liquids, thereby accounting for other damping sources. Neither the most viscous nor least viscous drops studied imposed the greatest damping on cantilever motion. Instead, drops of intermediate viscosity strike the most effective balance of sloshing and internal dissipative capacity. Very thin cantilevers with sloshing drops express more than one dominant frequency and vibrate erratically, often shifting phase, and presenting a challenge for quantification of damping. Finally, we introduce a new dimensionless group aimed at incorporating all salient variables of our cantilever-drop system.   

Droplet Impact on Flexible Mesoscale Pillars through Drop Tower Experimentation

Droplet impact on a substrate occurs in technologies such as fog harvesting and water recovery on spacecraft. Modifying the topology of a substrate may drastically change the outcomes of droplet impact and in turn improve the capture efficiency. An investigation into droplet impact on substrates with slender flexible millimetric pillars is carried out. The effects of pillar topology and stiffness on droplet impact and capture phenomena are considered. Pillar deflections are shown to alter the droplet impact as well as final equilibrium configurations. Dynamics of such drop impact are rationalized through drop tower experiments and simple models.   

An exposition of facemask efficacy against large size cough droplets

The usage of facemasks has been ubiquitously recommended worldwide as a physical barrier to the ejected droplet during respiratory events. This is an effective strategy for restricting various droplet-based disease transmission, as in the case of COVID-19. Although the N95 facemask has high efficacy against respiratory droplets, its accessibility/affordability for the general population is still deprived. As a possible solution, using a makeshift facemask (surgical or cotton facemasks) is generally advised by policymakers. Although such endorsement could be economical and accessible, quantitative analysis on the effectiveness of such facemasks is still lacking. Using a large-sized surrogate cough droplet, we identified an additional route of disease transmission, which involves atomization of large-sized cough droplets into numerous daughter droplets. It is shown that most of such atomized droplets are of sizes which is critical for aerosolization¹. This suggested that the amount of aerosol generated (thereby the risk of infection) through this mechanism is higher than the earlier predictions based on mask filtration efficiencies alone. A scaling argument based on the energy balance of impact dynamics was obtained and verified using experiments to identify a criterion for droplet penetration through a mask layer. The parametric analysis was also carried, which involves droplet impact velocities (corresponding to different respiratory events), impact angles (corresponding to different mask orientations), mask fabrics (surgical and cotton facemasks), and different washing cycles. The obtained results are discussed in detail, and a recommendation of the most suitable fabric for making homemade facemasks is presented.

 

References:

[1]       S. Sharma, R. Pinto, A. Saha, S. Chaudhuri, S. Basu, On secondary atomization and blockage of surrogate cough droplets in single- And multilayer face masks, in: Sci. Adv., 2021: pp. 1–26. https://doi.org/10.1126/sciadv.abf0452.