Drag reduction of up to 77.2% is achieved with air rings confined by alternating superhydrophobic and hydrophilic strips.

Coaxial jet electrospray is a technique to generate microencapsules, which uses electric forces to create a coaxial microjet from two immiscible liquids. Compound droplets with narrow size distribution are produced after the jet breaks up. In this paper, the spraying modes are investigated experimentally with proper flow rates of the inner and outer liquids. Ethanol/glycerol/tween mixture ͑outer liquid͒ and cooking oil ͑inner liquid͒ are fed into the gap between outer and inner capillaries and the inner capillary, respectively. The spraying modes presented in our experiments are "dripping mode," "dripping mode in spindle," "cone-jet mode," "pulse mode in cone," and "multijets mode" sequentially, as the applied voltage increases. The region of stable cone-jet mode extends with decrease of the outer liquid flow rate and increase of the inner one. It is found that the spray phenomena are mainly determined by properties of the outer liquid, which is viscous and electric conductive enough. A rudimentary physical model is developed, in which both the viscosity and liquid interface tension are taken into account.

The flapping coupling between two filaments is studied theoretically and experimentally in this paper. A temporal linear instability analysis is carried out based on a simplified hydrodynamic model. The dispersion relationship between the eigen-frequency ω and wavenumber k is expressed by a quartic equation. Two special cases of flapping coupling, i.e. two identical filaments having the same length and two filaments having different lengths, are studied in detail. In the case of two identical filaments, the theoretical analysis predicts four coupling modes, i.e. the stretched-straight mode, the antisymmetrical in-phase mode, the symmetrical out-of-phase mode and the indefinite mode. The theory also predicts the existence of an eigenfrequency jump during transition between the in-phase and out-of-phase modes, which has been observed in previous experiments and numerical simulations. In the case of two filaments having different lengths, four modes similar to those in the former case are identified theoretically. The distribution of coupling modes for both the cases is shown in two planes. One is a dimensionless plane of S vs. U, where S is the density ratio of solid filament to fluid and U2 is the ratio of fluid kinetic energy to solid elastic potential energy. The other is a dimensional plane of the half-distance (h) between two filaments vs. the filament length (L). Relevant experiments are carried out in a soap-film tunnel and the stable and unstable modes are observed. Theory and experiment are compared in detail. It should be noted that the model used in our analysis is a very simplified one that can provide intuitional analytical results of the coupling modes as well as their qualitative distributions. The factors neglected in our model, such as vortex shedding, viscous and nonlinear effects, do not allow the model to predict results precisely consistent with the experiments. Moreover, the Strouhal numbers of the flapping filaments are found to be generally around a fixed value in the experiments for both cases, implying that the filaments try to maintain a lower potential energy state.

The nanofluidic diode, as one of the emerging nanofluidic logic devices, has been used in many fields such as biosensors, energy harvesting, and so on. However, the entrance effects of the nanofluidic ionic conductance were less discussed, which can be a crucial factor for the ionic conduction. Here we calculate the ionic conductance as a function of the length-to-pore ratio (L/r), which has a clear boundary between nanopore (surface dominated) and nanochannel (geometry dominated) electrically in diluted salt solution. These entrance effects are even more obvious in the rectified ionic conduction with oppositely charged exterior surfaces of a nanopore. We build three models-Exterior Charged Surface model (ECS), Inner Charged Surface model (ICS), and All Charged Surface model (ACS)-to discuss the entrance effects on the ionic conduction. Our results demonstrate, for a thin nanopore, that the ECS model has a larger ionic rectification factor (Q) than that of ICS model, with a totally reversed tendency of Q compared to the ICS and ACS models as L/r increases. Our models predict an alternative option of building nanofluidic biosensors that only need to modify the exterior surface of a nanopore, avoiding the slow diffusion of molecules in the nanochannel.

The passive oscillations of flexible filaments in a flowing soap film were investigated to learn the serial interaction between them. When arranged in tandem, the downstream filament flaps at the same frequency as that of the upstream one, but with a larger amplitude, whereas the upstream one is almost unaffected compared to the single filament case. The data analysis shows the downstream filament indeed extracts energy from the vortex street and receives greater force than the upstream one or a single filament in a uniform flow.