Electrochromatographic separations of three metal complexes were achieved through observing the visualization evolution process of moving chelation boundaries (MCBs) on microfluidic paper-based analytical devices (μPADs). A simplified photolithography was developed to pattern the μPADs within 5–10 min with three steps only, manually gelatinize, expose and develop, through thinning SU-8 2150 with trichloromethane. With a cutting method, many bubbles were clearly observed in the single-component MCB system on the μPADs, but were not found with this photolithography method under the same voltages. It was demonstrated that this photolithography for patterning narrow paper channels rejected Joule heating more effectively than the cutting paper method did. It was observed simultaneously that the apparent distances of electrophoretic and electroosmotic migrations of the blue [Cu-EDTA]²⁻ chelation boundary increased with increasing electrical field strength and width of the paper channels, respectively. The higher the Cu²⁺ concentrations were, the slower the electrophoretic migrations of the blue boundary were, and the faster the electroosmotic migrations of the blue boundary were on the μPADs with this photolithography. The separations and concentrations of the coloured zones, pink [Co–EDTA]²⁻, blue [Cu–EDTA]²⁻ and yellow [Fe–EDTA]²⁻, were clearly observed in the applied electric fields on the μPADs. The studies benefit extending the application of μPADs as low-cost, disposable analysis tools and deepen understanding of the comprehensive mechanisms concerning MCBs, partition adsorption and electroosmotic flow on μPADs.

The heating mechanism of standing surface acoustic waves (SSAWs) on a LiNbO₃ substrate has been experimentally studied. Three devices with different substrates were used to heat the drops with NaCl concentrations ranging from 0 to 1 g/l, respectively. The device with a glass substrate was used to shield acoustic waves. The device with an Au layer between the LiNbO₃ substrate and the droplet was used to shield the alternating current field. The results show that the thermal effect induced by SSAWs on the LiNbO₃ substrate is composed of the acoustothermal effect due to SSAWs and the electric field thermal effect (Joule heat) due to the alternating current field. The electric field thermal effect which is ignored in SSAW devices previously plays an important role in the thermal effect induced by SSAWs. These results provide a meaningful insight into the mechanism of SSAW-based heating, which is of great help to guide the effective use of the SSAW-based heating technique for various applications.

The transverse skin of dolphins exhibits a remarkable drag reduction effect. Although previous studies have identified the drag reduction effect of transverse grooves, no clear guidelines exist regarding the impact of groove parameters on turbulent drag reduction. Hence, this paper suggests a novel numerical study using the Reynolds-averaged Navier–Stokes method to investigate the influence of half-sine wave structure parameters on turbulent drag. The results showed that the aspect ratio of shape parameters significantly affected the drag reduction rate by altering the flow velocity and drag distribution near the wall and increasing the viscous sublayer thickness. Moreover, a novel index friction pressure ratio FPr was introduced to evaluate the drag reduction. It was revealed that the ratio FPr was stable at optimal drag reduction effect regardless of the velocity. By optimizing the FPr, a maximum drag reduction of 29.3% was achieved. These findings provide insight for optimizing transverse groove in drag-reducing surface applications.

• Surface acoustic wave (SAW) technology is proposed to exfoliate large quantities and uniform layers of two-dimensional (2D) metal-organic framework (MOF) Zn₂(bim)₄ nanosheets. • The thickness of MOF sheet can be controlled by adjusting the exposure time of surface acoustic wave. • Electric field generated by surface acoustic waves plays an important role in the stripping process. Monolayer nanosheets can only be obtained under the simultaneous action of sound field and electric field.