Inverse problem of capillary filling.

The inverse problem of capillary filling, as defined in this work, consists in determining the capillary radius profile from experimental data of the meniscus position l as a function of time t. This problem is central in diverse applications, such as the characterization of nanopore arrays or the design of passive transport in microfluidics; it is mathematically ill posed and has multiple solutions; i.e., capillaries with different geometries may produce the same imbibition kinematics. Here a suitable approach is proposed to solve this problem, which is based on measuring the imbibition kinematics in both tube directions. Capillary filling experiments to validate the calculation were made in a wide range of length scales: glass capillaries with a radius of around 150 µm and anodized alumina membranes with a pores radius of around 30 nm were used. The proposed method was successful in identifying the radius profile in both systems. Fundamental aspects also emerge in this study, notably the fact that the l(t)∝t1/2 kinematics (Lucas-Washburn relation) is not exclusive of uniform cross-sectional capillaries.

Real-time study of protein adsorption kinetics in porous silicon.

This paper presents an optical method for real-time monitoring of protein adsorption using porous silicon self-supported microcavities as a label-free detection platform. The study combines an experimental approach with a physical model for the adsorption process. The proposed model agrees well with experimental observations, and provides information about the kinetics of diffusion and adsorption of proteins within the pores, which will be useful for future experimental designs.

Optofluidic characterization of nanoporous membranes.

An optofluidic method that accurately identifies the internal geometry of nanochannel arrays is presented. It is based on the dynamics of capillary-driven fluid imbibition, which is followed by laser interferometry. Conical nanochannel arrays in anodized alumina are investigated, which present an asymmetry of the filling times measured from different sides of the membrane. It is demonstrated by theory and experiments that the capillary filling asymmetry only depends on the ratio H of the inlet to outlet pore radii and that the ratio of filling times vary closely as H(7/3). Besides, the capillary filling of conical channels exhibits striking results in comparison to the corresponding cylindrical channels. Apart from these novel results in nanoscale fluid dynamics, the whole method discussed here serves as a characterization technique for nanoporous membranes.

Current-voltage characteristics in macroporous silicon/SiOx/SnO2:F heterojunctions.

We study the electrical characteristics of macroporous silicon/transparent conductor oxide junctions obtained by the deposition of fluorine doped-SnO2 onto macroporous silicon thin films using the spray pyrolysis technique. Macroporous silicon was prepared by the electrochemical anodization of a silicon wafer to produce pore sizes ranging between 0.9 to 1.2 µm in diameter. Scanning electronic microscopy was performed to confirm the pore filling and surface coverage. The transport of charge carriers through the interface was studied by measuring the current-voltage curves in the dark and under illumination. In the best configuration, we obtain a modest open-circuit voltage of about 70 mV and a short-circuit current of 3.5 mA/cm2 at an illumination of 110 mW/cm2. In order to analyze the effects of the illumination on the electrical properties of the junction, we proposed a model of two opposing diodes, each one associated with an independent current source. We obtain a good accordance between the experimental data and the model. The current-voltage curves in illuminated conditions are well fitted with the same parameters obtained in the dark where only the photocurrent intensities in the diodes are free parameters.

Capillary filling in nanostructured porous silicon.

An experimental study on the capillary filling of nanoporous silicon with different fluids is presented. Thin nanoporous membranes were obtained by electrochemical anodization, and the filling dynamics was measured by laser interferometry, taking advantage of the optical properties of the system, related with the small pore radius in comparison to light wavelength. This optical technique is relatively simple to implement and yields highly reproducible data. A fluid dynamic model for the filling process is also proposed including the main characteristics of the porous matrix (tortuosity, average hydraulic radius). The model was tested for different ambient pressures, porous layer morphology, and fluid properties. It was found that the model reproduces well the experimental data according to the different conditions. The predicted pore radii quantitatively agree with the image information from scanning electron microscopy. This technique can be readily used as nanofluidic sensor to determine fluid properties such as viscosity and surface tension of a small sample of liquid. Besides, the whole method can be suitable to characterize a porous matrix.