Session L08: Bubbles, Surfactants and Foams

Coalescence Induced Self-Propelled Detachment of Surface Bubbles

The dynamics of nucleation, growth, and detachment of gas bubbles has important implications for many catalytic and electrochemical gas evolution reactions in liquids. In the current work, we experimentally and theoretically examine the growth and detachment dynamics of oxygen bubbles from hydrogen peroxide decomposition catalyzed by gold. Bubbles are demonstrated to grow from an oxygen-oversaturated environment. The dynamical evolution of bubbles is influenced by comprehensive effects combining chemical catalysis and physical mass transfer. Self-propelled detachment is visualized and demonstrated to be induced by the coalescence of two bubbles, where the released surface energy upon coalescence is converted to the kinetic energy of the out-of-plane jumping motion of the merged bubble. A scaling dependence of the critical parent bubble size for the jumping behavior is theoretically derived and found to be in agreement with the experimental results. Energy balances reveal that the parent size inequality not only influences the critical size for jumping, but determines how much energy can be provided for the jumping motion. The current results provide both physical insight for the bubble interactions and practical strategies for applications in chemical engineering.
  

The nucleation rate of single O2 nanobubbles on Pt nanoelectrodes

Nanobubble nucleation is a problem that affects efficiency in electrocatalytic reactions, since those bubbles can block the surface of the catalytic sites. We focus on the nucleation rate of O2 nanobubbles resulting from electrooxidation of H2O2 at Pt disk nanoelectrodes. By applying a critical peak current, inbp, bubbles form almost instantaneously. However, for lower currents, bubble nucleation is a stochastic process in which the nucleation time, tind, dramatically decreases as the applied current approaches inbp, a consequence of the local supersaturation level, ζ, increasing at high currents. This fact provides a means to measure the stochastic tind. We study in detail the different conditions in which nanobubbles appear, concluding that the electrode surface needs to be pre-conditioned for achieving reproducible results. We also measure the activation energy for bubble nucleation, Ea and we determine the footprint diameter L=8-15 nm, the contact angle to the electrode surface θ=135-155o and the number of O2 molecules contained in the nucleus (50 to 900 molecules), assuming that the nucleus has a spherical cap shape.   

Evaporation-induced foam stabilization in blended lubricants

Mitigating lubricant foaming is of primary concern to lubricant manufacturers, especially in the context of high performance machinery such as wind turbines. In this study, we explored foaming in blended lubricant base oils. Utilizing single bubble experiments coupled with interferometry, we found that blending lubricant base oils exacerbates the foamability of the resulting mixture. This increased bubble stability is a consequence of Marangoni flows driven by the differential evaporation of various components in the blended system. Interestingly, the resulting flows are also seen to cause the bubble wall thickness to ‘pulse’ (spontaneous dimpling - O. Velev et.al, JCIS 1993), resulting from the interplay between capillary forces draining the bubble and Marangoni flows entraining liquid into the bubble. As the blending of lubricant oils is important in formulation, we utilized silicone oil mixtures to further characterize the evaporation-induced bubble stabilization as a function of the Marangoni number and the dimensionless dimple formation number (C. A Miller et.al, JCIS 1997).   
  

Mechanics of Active Foam: Local Energy Injection in an Addressable 2D Foam

The study of foam has inspired many insights into cellularized materials, including biological tissue. While cellular sheets superficially resemble passive foam structures, cells incorporate many layers of internal activity and feedback. To the best of our knowledge, no abiotic “active” foam experimental systems currently exist. Such a material is developed here, where activity of a single voxel is driven by volume oscillations in an addressable foam. The platform allows air to be periodically injected and removed from individual voxels in a 2D soap foam in a Hele-Shaw cell. This cyclic energy injection leads to fascinating dynamics involving neighbor-swapping events (T1) including cascades and global dynamics. We quantify this response of a single active voxel as a function of energy injected, symmetry and time course of evolution. Next, we study interactions between multiple active voxels with cyclic perturbations (in-phase and out-of-phase). Stroboscopic analysis is used when studying cyclic forcing. Just like our understanding of active fluids and active solids has brought new light to a multitude of problems - active foam provides a new platform to ask questions both in foam physics and, by analogy, in tissue mechanics.   

Fracture dynamics in foam: Finite-size effects

The injection of gas under a defined pressure difference and high enough rate into a layer of quasi-two-dimensional foam bubbles induces ruptures of successive films that travel steadily at well-defined speed. While this process analogous to type-I fracture propagation can be described successfully in its salient features by simple one-dimensional models, quantitative discrepancies to experiment remain. Within a two-dimensional network model incorporating fluid dynamical processes of film instabilities, shear dissipation, and deformation of free interfaces, we find that the dimensions of the foam sample significantly affect the speed of the cracks as well as the pressure necessary to sustain them: cracks in wider samples travel faster at a given driving stress, but are able to avoid arrest and maintain propagation at a lower pressure (the velocity gap becomes smaller). The system thus becomes a study case for stress concentration and the transition between discrete and continuum systems in dynamical fracture; taking into account the finite dimensions of the system improves agreement with experiment.

  

Formation, growth and coalescence of nanoscopic mesas in stratifying freestanding films formed with micellar solutions of ionic surfactants

Thin liquid films containing micelles, nanoparticles, polyelectrolyte-surfactant complexes and smectic liquid crystals undergo thinning in a discontinuous, step-wise fashion. The discontinuous jumps in thickness are often characterized by quantifying changes in the intensity of reflected monochromatic light, modulated by thin film interference from a region of interest. Stratifying thin films exhibit a mosaic pattern in reflected white light microscopy, attributed to the coexistence of domains with various thicknesses, separated by steps. Using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols we recently developed, we spatially resolve for the nanoscopic landscape of stratifying freestanding thin films. We characterize discontinuous, thickness transitions with concentration-dependent steps of 5-25 nm, and show that both flat and non-flat features are sculpted by oscillatory, periodic, supramolecular structural forces that arise in confined fluids.  In particular, for thin films containing micelles of an anionic surfactant sodium dodecyl sulfate (SDS),  we analyze the formation and coalescence of nanoscopic mesas, that spontaneously appear around expanding thinner darker domains.