Role of Grounded Liquid Collectors in Precise Patterning of Electrospun Nanofiber Mats.

Liquid collectors are applicable as ground collectors in electrospinning, which fabricates complex nanofiber architectures. However, the influence of the electrical properties of liquid collectors on the controlled deposition of electrospun nanofiber mats has received little attention. Here, we prepare two types of liquid collectors (electrolyte solutions and dielectric liquids) and newly scrutinize their roles in the patterning of electrospun nanofiber mats in experiments and in numerical simulations. By simulating the concentrations of the electric fields around the liquid collectors, we indirectly evaluated the patternability of the collectors. The patternability trends were verified by the patterning of nanofiber mats on line-array-shaped liquid collectors fabricated by electrospinning. The deposition accuracy of the electrolyte solution collector was very high, equivalent to that of a conventional metal collector even at low salt concentrations (e.g., 0.01 M KCl). However, the nanofiber mats fabricated by electrospinning with the dielectric liquid collector showed retarded patternability.

Dual-mode reconfigurable focusing using the interface of aqueous and dielectric liquids.

An optofluidic lens serves as a highly reconfigurable device to manipulate light by using a smoothly curved interface between immiscible liquids. Here we propose a dielectro-optofluidic lens (DOL) that is capable of dual-mode reconfigurable focusing. In this DOL, light focuses through a dielectric liquid-aqueous liquid interface where movement and deformation of the interface are achieved by electrohydrodynamic (EHD) actuation. We initially perform alternating current-EHD actuation of the dielectric liquid to obtain its benefit of frequency-dependent behavior and to prevent electrolysis of the aqueous liquid. Our DOL uniquely operates in two modes, namely, an oscillation mode in the low-frequency regime (<1 Hz) with a 10 mm focus-tuning range and a static mode in the high-frequency regime (>10 Hz) with a 1 mm focus-tuning range, which are easily modulated on demand by the frequency range of the applied voltage. We successfully conduct proof-of-concept experiments, including extending the depth-of-field using the oscillation mode to clearly visualize thick targets, and integrating the proposed DOL with a photoacoustic microscope using the static mode to adjust the focal point.

Wide and fast focus-tunable dielectro-optofluidic lens via pinning of the interface of aqueous and dielectric liquids.

Electrohydrodynamic actuation of dielectric liquid enables the development of an efficient focus-tunable dielectro-optofluidic lens (DOL) by manipulating a liquid-liquid interface. However, practical utilization of the previous DOL is hindered by its narrow and slow focus-tunability due to the direct movement of the interface. Here, we propose pinning the interface to directly change the interface shape while preventing the interface movement. The newly designed DOL exploits sudden changes in the channel diameter and the surface wettability to firmly pin the interface. Our results demonstrate that the tuning range of the DOL from -40 to +35 diopters is achieved in 0.1 s.

Modulating wall shear stress gradient via equilateral triangular channel for in situ cellular adhesion assay.

This study introduces an equilateral triangular channel (ETRIC), a novel microfluidic channel with an equilateral triangular cross-section, for cell adhesion assay by modulating the wall shear stress (WSS) gradient. The channel can generate a parabolic WSS gradient perpendicular to the flow direction at a single flow rate, and cell detachment can be in situ screened in response to spatially different levels of WSS. The existence of a simple form of exact solution for the velocity field inside the entire ETRIC region enables the easy design and modulation of the WSS levels at the bottom surface; therefore, the detachment of the cells can be investigated at the pre-defined observation window in real time. The exact solution for the velocity field was validated by comparing the analytical velocity profile with those obtained from both numerical simulation and experimental particle image velocimetry. The parabolic WSS gradient can be generated stably and consistently over time at a steady-state condition and easily modulated by changing the flow rate for the given ETRIC geometry. The WSS gradient in the ETRIC is in a symmetric parabolic form, and this symmetry feature doubles the experimental data, thereby efficiently minimizing the number of experiments. Finally, a WSS gradient ranging from 0 to 160 dyn/cm2 was generated through the present ETRIC, which enables not only to measure the adhesion strength but also to investigate the time-dependent detachment of NIH-3T3 cells attached on the glass.

Gradient shadow pattern reveals refractive index of liquid.

We propose a simple method that uses a gradient shadow pattern (GSP) to measure the refractive index nL of liquids. A light source generates a "dark-bright-dark" GSP when it is projected through through the back of a transparent, rectangular block with a cylindrical chamber that is filled with a liquid sample. We found that there is a linear relationship between nL and the proportion of the bright region in a GSP, which provides the basic principle of the proposed method. A wide range 1.33 ≤ nL ≤ 1.46 of liquids was measured in the single measurement setup with error <0.01. The proposed method is simple but robust to illuminating conditions, and does not require for any expensive or precise optical components, so we expect that it will be useful in many portable measurement systems that use nL to estimate attributes of liquid samples.