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The subject of viscoelastic flow phenomena is crucial to many areas of engineering and the physical sciences. Although much of our understanding of viscoelastic flow features stems from carefully designed experiments, preparation of model viscoelastic fluids remains a challenge; for example, fabricating a series of fluids with different fluid shear moduli G0, but with an identical relaxation time τ, is nontrivial. In this work, we harness the nonideality of nearly constant-viscosity elastic fluids, commonly known as “Boger fluids,” made with polyisobutylene (PIB), to develop an experimental methodology that produces a set of fluids with desired viscoelastic properties, specifically, G0, τ, and the first normal stress difference coefficient ψ1. Through a linear algebraic relation between the rheological properties of interest (G0, τ, ψ1) and the fluid compositions in terms of polymer concentration c, molecular weight M, and solvent viscosity ηs, we developed a “design equation” that takes G0, τ, ψ1 as inputs and calculates values for c, M, ηs as outputs. Using this method, fabrication of dilute viscoelastic fluids whose rheological properties are a priori known can be achieved. 

Physical Review . Polynyas—persistent regions of open water within polar sea ice—play a critical role in polar ocean-atmosphere interactions. We combine theoretical modeling and numerical simulations to investigate the dynamics and thermodynamics of wind-driven, latent-heat-generated polynya formation adjacent to straight and curved coastlines. Under the assumption of negligible ice internal pressure, we propose a one-dimensional, continuum, mass- and momentum-conserving theory characterizing the offshore distribution of ice velocity and the spatiotemporal evolution of ice concentration. Finite-element simulations incorporating realistic sea-ice rheology validate the theoretical predictions, demonstrating strong agreement in steady-state polynya widths and ice dynamics. These results align qualitatively with observational climate data. Furthermore, we generalize the framework to two dimensions, enabling quantitative predictions of leeward polynya formation around a model circular island. The proposed theoretical framework advances mechanistic understanding of polynya formation and provides a foundation for improving their representation in climate models. 

Rapid Room-Temperature Aerosol Dehydration (RTAD) is a novel, scalable drying technology for powderization and thermal stabilization of pharmaceutical drug products. Compared to conventional spray drying processes, typically using droplets of 10–200 lm in diameter generated by high-shear spraying, RTAD uses much smaller droplets with diameter 0.1 to 20 lm produced in modified liquid atomization processes. These fine droplets evaporate rapidly within 10–100 ms under room-temperature conditions, thereby reducing drying-induced stresses for thermally sensitive biologics. In this study, we used Green Fluorescent Protein (GFP) as a model biological molecule to optimize the design of the RTAD system and the process parameters. We experimentally investigated the effects of droplet size, multiphase flow patterns in the drying chamber, and application of polysorbate 20 as a model surfactant on GFP fluorescence after drying and powder reconstitution. The experiments demonstrated that the presence of surfactant in the formulation significantly influenced the GFP fluorescence intensity, especially for smaller droplets. The numerical studies using Computational Fluid Dynamics simulations revealed that the drying of droplets was dependent on the patterns of multiphase flow in the drying chamber, which can impact the intensity of GFP fluorescence in the produced dry powders. Non-axisymmetric flows and closed circulating streamlines near the drying gas inlet resulted in considerably longer particle residence times, which we infer means that GFP molecules were subjected to excess stress that negatively impacted the GFP fluorescence intensity. Through iterative optimization of the chamber design, process parameters and feedstock formulation, we achieved recovery of the GFP fluorescence intensity that exceeded 96% in the obtained dry powders. This work establishes GFP as a sensitive model biologic and its fluorescence intensity as a powerful tool to rapidly assess process efficiency and the ability to preserve bioactivity after dehydration. The study has broad implications for the design and scale-up of drying technologies, which can potentially transform the production of dry powder biopharmaceuticals. 

Abstract Despite centuries of investigation, bubbles continue to unveil intriguing dynamics relevant to a multitude of practical applications, including industrial, biological, geophysical, and medical settings. Here we introduce bubbles that spontaneously start to ‘gallop’ along horizontal surfaces inside a vertically-vibrated fluid chamber, self-propelled by a resonant interaction between their shape oscillation modes. These active bubbles exhibit distinct trajectory regimes, including rectilinear, orbital, and run-and-tumble motions, which can be tuned dynamically via the external forcing. Through periodic body deformations, galloping bubbles swim leveraging inertial forces rather than vortex shedding, enabling them to maneuver even when viscous traction is not viable. The galloping symmetry breaking provides a robust self-propulsion mechanism, arising in bubbles whether separated from the wall by a liquid film or directly attached to it, and is captured by a minimal oscillator model, highlighting its universality. Through proof-of-concept demonstrations, we showcase the technological potential of the galloping locomotion for applications involving bubble generation and removal, transport and sorting, navigating complex fluid networks, and surface cleaning. The rich dynamics of galloping bubbles suggest exciting opportunities in heat transfer, microfluidic transport, probing and cleaning, bubble-based computing, soft robotics, and active matter. 

The dynamics of self-propelled colloidal particles is strongly influenced by their environment through hydrodynamic and, in many cases, chemical interactions. We develop a theoretical framework to describe the motion of confined active particles by combining the Lorentz reciprocal theorem with a Galerkin discretisation of surface fields, yielding an equation of motion that efficiently captures self-propulsion without requiring an explicit solution for the bulk fluid flow. Applying this framework, we identify and characterise the long-time behaviours of a Janus particle near rigid, permeable and fluid–fluid interfaces, revealing distinct motility regimes, including surface-bound skating, stable hovering and chemo-hydrodynamic reflection. Our results demonstrate how the solute permeability and the viscosity contrast of the surface influence a particle’s dynamics, providing valuable insights into experimentally relevant guidance mechanisms for autophoretic particles. The computational efficiency of our method makes it particularly well suited for systematic parameter sweeps, offering a powerful tool for mapping the phase space of confined active particles and informing high-fidelity numerical simulations. 

The Weissenberg effect, or rod-climbing phenomenon, occurs in non-Newtonian fluids where the fluid interface ascends along a rotating rod. Despite its prominence, theoretical insights into this phenomenon remain limited. In earlier work, Joseph & Fosdick (1973,Arch. Rat. Mech. Anal.vol. 49, pp. 321–380) employed domain perturbation methods for second-order fluids to determine the equilibrium interface height by expanding solutions based on the rotation speed. In this work, we investigate the time-dependent interface height through asymptotic analysis with dimensionless variables and equations using the Giesekus model. We begin by neglecting inertia to focus on the interaction between gravity, viscoelasticity and surface tension. In the small-deformation scenario, the governing equations indicate the presence of a boundary layer in time, where the interface rises rapidly over a short time scale before gradually approaching a steady state. By employing a stretched time variable, we derive the transient velocity field and corresponding interface shape on this short time scale, and recover the steady-state shape on a longer time scale. In contrast to the work of Joseph and Fosdick, which used the method of successive approximations to determine the steady shape of the interface, we explicitly derive the interface shape for both steady and transient cases. Subsequently, we reintroduce small but finite inertial effects to investigate their interaction with viscoelasticity, and propose a criterion for determining the conditions under which rod climbing occurs. Through numerical computations, we obtain the transient interface shapes, highlighting the interplay between time-dependent viscoelastic and inertial effects. 

This paper is associated with a video winner of a 2024 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion,