We theoretically illustrate how complex fluids flowing over superhydrophobic surfaces may exhibit giant flow enhancements in the double limit of small solid fractions ($\epsilon \ll 1$) and strong shear thinning ($\beta \ll 1, \beta$ being the ratio of the viscosity at infinite shear rate to that at zero shear rate). Considering a Carreau liquid within the canonical scenario of longitudinal shear-driven flow over a grooved superhydrophobic surface, we show that, as $\beta$ is decreased, the scaling of the effective slip length at small solid fractions is enhanced from the logarithmic scaling $ln(1/\epsilon )$ for Newtonian fluids to the algebraic scaling $1/{\epsilon }^{\frac{1-n}{n}}$, attained for $\beta =\mathcal{O}\left({\epsilon }^{\frac{1-n}{n}}\right), n\in (0,1)$ being the exponent in the Carreau model. We illuminate this scaling enhancement and the geometric-rheological mechanism underlying it through asymptotic arguments and numerical simulations.

The aerodynamic breakup of a polymeric droplet in the bag breakup regime is investigated experimentally and compared with the result of the Newtonian droplet. To understand the effect of liquid elasticity, the Weber number is kept fixed ($\approx 15$) while the elasticity number is varied in the range of $\sim {10}^{-4}–{10}^{-2}$. Experiments are performed by allowing a liquid droplet to fall in a horizontal, continuously flowing air stream. It is observed that the initial deformation dynamics of a polymeric droplet is similar to the Newtonian droplet. However, in the later stages, the actual fragmentation of liquid mass is resisted by the presence of polymers. Depending upon the liquid elasticity, fragmentation can be completely inhibited in the timescale of experimental observation. We provide a framework to study this problem, identify the stages where the role of liquid elasticity can be neglected and where it must be considered, and finally, establish a criterion that governs the occurrence or the absence of fragmentation in a specified time period.

The evolution and coverage of bubbles resulting from gas evolution at the reaction interface significantly impact mass transfer and the flow field within the photoelectrochemical water splitting system, posing a significant obstacle to overcoming the efficiency bottleneck of photoelectrochemical water splitting. In situ bubble observation can be realized via a coupled experimental system combining electrochemical measurements and high-speed imaging. The results indicate that bubble growth behavior correlates with system temperature and operating voltage; that is, the bubbles grow in an oscillating state on the electrode surface at the low operating voltage and high system temperature. Besides, the deviation amplitude of oscillating bubbles increases with system temperature yet decreases with operating voltage. The results indicate that the time coefficient of the growing bubble decreases, while the growth rate and gas evolution efficiency of the bubble increase with the increase of oscillating amplitude in the horizontal direction. The study of dynamic behaviors between the oscillating bubble and the contacted electrode reveals that the bubble oscillation is characterized by the periodic contraction and extension of the triple line. The mass transfer model of concentration boundary layer and the force model of oscillating bubble are constructed based on the interaction between the bubbles and the surrounding flow field. The mass transfer model has considered the microconvection caused by single-phase natural convection, gas-liquid interface expansion, and the oscillation of bubbles. Then through the force balance model, the solutal Marangoni force is found to gradually increase as the contact line extends outward.

We investigate the impact of a single miniature Helmholtz resonator on wall-bounded turbulence using time-resolved planar particle image velocimetry. A particular aim is to explain the mechanism by which a resonator alters the turbulent velocity fluctuations of different scales. A grazing flow configuration is studied in which the resonator is embedded in the wall beneath a turbulent boundary layer at a friction Reynolds number of ${\text{Re}}_{\tau }\approx 2300$; the resonator is designed so that its resonance frequency matches the peak frequency of the wall-pressure spectrum. It is found that the resonator amplifies velocity fluctuations near its resonance frequency, while it attenuates the energy of subresonance scales. Underlying mechanisms responsible for these changes in energy are discussed in view of the resonator's local impedance condition. It is posited that large-scale velocity fluctuations in the wall-normal velocity, at temporal frequencies below resonance, are subject to a phase-opposed wall-normal velocity perturbation when the TBL flow convects over the resonator's orifice. This yields a decrease of large-scale energy in $\overline{u^{\prime }{u}^{\prime }}, -\overline{u^{\prime }{v}^{\prime }}$, and $\overline{v^{\prime }{v}^{\prime }}$. In addition, modifications of the wall-shear stress field downstream of the resonator are addressed. Insights from this research will contribute to the development of surface designs for passive skin-friction control using arrays of miniature resonators.

This paper is associated with a poster winner of a 2023 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 poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.P0018

This paper is associated with a poster winner of a 2023 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 poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.P0008.

This paper is associated with a video winner of a 2023 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, https://doi.org/10.1103/APS.DFD.2023.GFM.V0030

This paper is associated with a poster winner of a 2023 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.P0019.

This paper is associated with a video winner of a 2023 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.V0002

This paper is associated with a video winner of a 2023 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.V0079.

This paper is associated with a poster winner of a 2023 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 poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.P0005

This paper is associated with a video winner of a 2023 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, https://doi.org/10.1103/APS.DFD.2023.GFM.V0048 marker GFM.

This paper is associated with a poster winner of a 2023 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.P0011.

This paper is associated with a video winner of the 2023 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.V0087.

This paper is associated with a video winner of a 2023 American Physical Society's Division of Fluids 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, https://doi.org/10.1103/APS.DFD.2023.GFM.V0019

This paper is associated with a poster winner of a 2023 American Physical Society Division of Fluid Dynamics (DFD) Milton Van Dyke Award for the work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2023.GFM.P0030.

This paper numerically investigates droplet impact dynamics on solid substrates characterized by hydrophobic contact angles under the incompressible limit for both gas and liquid flow regimes. A phase diagram is constructed in the Weber number and contact angle space, revealing four distinct jet types with the emergence of a category termed “atomized jet” in the low-speed and small-contact-angle regime. Atomized jets exhibit unique characteristics, featuring smaller droplets and higher droplet velocities compared to conventional jet types. Comparative analysis between atomized jets and singular jets elucidates their differing formation mechanisms, with atomized jets predominantly forming from the retraction and collision of annular droplets, whereas singular jets originate from the rupture of the bottom liquid film. Additionally, the study highlights the role of surface tension in the vanishing of central cavities for atomized jets, contrasting with the dominance of inertial forces for singular jets with superhydrophobic contact angles. Examination of jet size and velocity distributions under varying contact angles reveals distinct trends, especially higher jet velocities and smaller jet sizes in the small contact angle region. Notably, the velocity trend in atomized jets differs from that of singular jets, exhibiting the highest velocities at the lowest Weber numbers, followed by a gradual decrease. These findings offer valuable insights into the complex dynamics of droplet impact and provide a comprehensive understanding of the influence of contact angles on jet formation and behavior.

When granular materials of shape-anisotropic grains are sheared in a split-bottom shear cell, a localized shear band is formed with a depression at its center. This effect is closely related to the alignment of the particles with the increase in aspect ratio, which, in turn, influences the local packing density, the stress distribution, and the system's overall bulk rheology. Particles with large aspect ratio tend to align with the shear direction, which increases the packing density in the shear band and affects rheological properties like stress, macroscopic friction coefficient, and effective viscosity. A scaling law correlates particle aspect ratio to macroscopic friction and effective viscosity, revealing shear-thinning behavior in bulk and near the surface.

A nematic liquid crystal, a phase of matter composed of rodlike molecules, exhibits a tendency towards uniform molecular alignment. Bodies inserted into such a fluid can disturb this orientational order, resulting in elastic stresses bound in the bulk fluid and exerted on the body surfaces, even at equilibrium. One avenue of energy relaxation is by a change in the immersed particle positions and orientations, leading to elastic forces and torques that bodies exert on each other through the fluid. Soft particles offer an additional means of relaxation, deformation, which in turn can modify particle interactions through the LC medium. We review classical work on rigid particles and their interactions in nematic liquid crystals and more recent work on the shapes and interactions of deformable bodies in LCs. In addition to experimental findings, a number of common mathematical modeling techniques, both in terms of a director field $\mathit{n}$ and a second-order tensor field $\mathbf{Q}$, are introduced throughout, including derivations of body forces and torques and, of particular use in the study of deformable media in LCs, surface tractions and moments.