(319a) Symmetry Breaking, Symmetry Recovery and Chaos in Evaporation Driven Marangoni Flows over Bubbles

Understanding the flow dynamics inside thin liquid films that make up bubbles is of practical and fundamental interest. Practically, this understanding is crucial for tuning bubble stability, while fundamentally, flows inside thin films are an excellent platform to study characteristics of 2D flows. Here we experimentally study the spatial and temporal dynamics of film thickness profiles over bubbles in binary liquid mixtures subjected to evaporation driven Marangoni flows. Initially, we demonstrate how bubble stability can be dramatically tuned with the help of evaporation driven flows. Subsequently, we probe the spatial structure and show that the spatial symmetry of the film thickness profiles are non-monotonic functions of initial volatile species concentration (c₀), with profiles being axisymmetric for both very low (1%) and very high (95%) concentrations. The temporal structure of the film thickness fluctuations over space reveal a similar non-monotonic dependence between species concentration and the spatial prevalence of fluctuation stochasticity. At 50% volatile species concentration, there is a complete breakdown of spatial symmetry and the film thickness fluctuations are chaotic everywhere in space, with the fluctuation statistics becoming spatially invariant and ergodic. For these cases, the power spectrum of thickness fluctuations forced by evaporation are shown to follow the Kolmogorov -5/3 scaling - a first such demonstration for forcing by evaporation. The observed non-monotonic behavior is a result of the system sensitivity to ambient disturbances scaling as (∂γ/∂γ)c₀(1-c₀)/μ, where ∂γ/∂γ is the solutal coefficient of surface tension and μ is the dynamic viscosity. These insights into evaporation driven Marangoni flows along with the reported experimental setup and protocols also serve as an excellent platform to further investigate externally forced 2D chaotic flows.