Flexible actuation of droplets is crucial for biomedical and industrial applications. Hence, various approaches using optical, electrical, and magnetic forces have been exploited to actuate droplets. For broad applicability, an ideal approach should be programmable and be able to actuate droplets of arbitrary size and composition. Here we present an “additive-free” magnetic actuation method to programmably manipulate droplets of water, organic, and biological fluids of arbitrary composition, as well as solid samples, on a ferrofluid-infused porous surface. We specifically exploit the spontaneously formed ferrofluid wetting ridges to actuate droplets using spatially varying magnetic fields. We demonstrate programmed processing and analysis of biological samples in individual drops as well as the collective actuation of large ensembles of micrometer-sized droplets. Such model respiratory droplets can be accumulated for improved quantitative and sensitive bioanalysis - an otherwise prohibitively difficult task that may be useful in tracking coronavirus.

Biological ion pumps selectively transport target ions against the concentration gradient, a process that is crucial to maintaining the out-of-equilibrium states of cells. Building an ion pump with ion selectivity has been challenging. Here we show that a Ti 3 C 2 T x MXene film suspended in air with a trapezoidal shape spontaneously pumps K + ions from the base end to the tip end and exhibits a K + / Na + selectivity of 4. Such a phenomenon is attributed to a range of properties of MXene. Thanks to the high stability of MXene in water and the dynamic equilibrium between evaporation and swelling, the film keeps a narrow interlayer spacing of ∼0.3 nm when its two ends are connected to reservoirs. Because of the polar electrical structure and hydrophilicity of the MXene nanosheet, K + ions experience a low energy barrier of ∼4.6 k B T when entering these narrow interlayer spacings. Through quantitative simulations and consistent experimental results, we further show that the narrow spacings exhibit a higher energy barrier to Na + , resulting in K + /Na + selectivity. Finally, we show that the spontaneous ion transport is driven by the asymmetric evaporation of the interlayer water across the film, a mechanism that is similar to pressure driven streaming current. This work shows how ion transport properties can be facilely manipulated by tuning the macroscopic shape of nanofluidic materials, which may attract interest in the interface of kirigami technologies and nanofluidics and show potential in energy and separation applications.

Nanofluidic membranes have shown great promise in harvesting osmotic energy but its scalablity remains challenging since most studies only tested with a membrane area of ≈10−2 mm2 or smaller. We demonstrate that metal‐organic‐framework membranes with subnanometer pores can be used for scalable osmotic power generation from hypersaline water sources. Our membrane can be scaled up to a few mm2, and the power density can be stabilized at 1.7 W m−2. We reveal that the key is to improve the out‐of‐membrane conductance while keeping the membrane's charge selectivity, contradicting the previous conception that the ionic conductivity of the membrane plays the dominating role. We highlight that subnanometer pores are essential to ensure the charge selectivity in hypersaline water sources. Our results suggest the importance to engineer the interplay between the in‐membrane and out‐of‐membrane ion transport properties for scalable osmotic power generation.