Digital holographic microscopy implementation for capillary filling measurements in nanoporous materials.

We report the implementation of lensless off-axis digital holographic microscopy as a non-destructive optical analyzer for nano-scale structures. The measurement capacity of the system was validated by analyzing the topography of a metallic grid with ≈150nm thick opaque features. In addition, an experimental configuration of self-reference was included to study the dynamics of the capillary filling phenomena in nanostructured porous silicon. The fluid front position as a function of time was extracted from the holograms, and the typical square root of time kinematics was recovered. The results shown are in agreement with previous works on capillary imbibition in similar structures and confirm a first step towards unifying holographic methods with fluid dynamics theory to develop a spatially resolved capillary tomography system for nanoporous materials characterization.

Sliding of drops on mesoporous thin films.

There is great interest in developing surfaces with enhanced properties for the sliding of liquid droplets. Here we show that both water and oil droplets placed on mesoporous thin film surfaces slide at relatively small tilt angles with respect to non-porous surfaces of the same material. The effect arises from a particular soft pinning at the contact line, which is a consequence of the fact that sessile droplets are partially "floating" onto a locally self-imbibed mesoporous film. Therefore, droplets present a reduced sliding angle and an enhanced sliding velocity in comparison to droplets on non-porous surfaces of the same material. The formed droplet-substrate interface is different to those observed on superhydrophobic or oil-infused surfaces, and involves a particular sliding dynamic. These findings would help to improve technical developments that require the precise handling of droplet mobility, whose interest span from chemical and biological assays in open microfluidic platforms to applications in energy and the environment.

Electrical current nanogeneration driven by spontaneous nanofluidic oscillations.

Exploiting natural phenomena is a central route for providing electricity to sustainably drive wearable electronics. Here we report a nano-scale water-driven energy generator that produces tiny electrical currents from spontaneous wetting-drying oscillations in mesoporous thin films. The system was fabricated with a wormlike mesoporous silica film, which was packed in between Cu and silicon contacts. The nanogenerator runs autonomously when a water droplet is laid over the film close to the Cu electrode, as water infiltration into the film under the electrode produces a direct-current. Wetting-drying cycles, which are spontaneously triggered by water evaporation, are perfectly correlated to the generated electrical current. The autonomous water displacement through the film yields a sustained energy conversion until the droplet reservoir vanishes. This novel water-driven nanogenerator opens new alternatives for versatile, mobile and cost-effective self-powering of nanosystems and nanodevices.

Configurable 2D nano-flows in mesoporous films using paper patches.

Designing and controlling spontaneous imbibition is becoming a key requirement for advanced devices, presenting a substantial scientific and engineering challenge. Here we describe an approach that allows directional imbibition into designed geometries. A set of custom domains based on paper microfluidics mold nano-imbibition in user-defined shapes such as curvatures, corners, and vertices into mesoporous thin films; enabling localized chemical reactions with programmable designs. The method also achieves nano-size filtration, allows the generation and delivery of reagent gradients in a nanofluidic fashion, and it can be used as a reactor for the synthesis of patterned metallic nanoparticle arrays. By using this easy-to-build hybrid platform, users can create functional nanofluidic domains in custom geometries and perform spatially shaped chemistry. The ability to integrate mesoporous nanofluidic generation and paper-based microfluidics has made the hybrid system an exciting candidate for versatile nanoflow applications.