Thanh-Vinh Nguyen

Depinning-Induced Capillary Wave during the Sliding of a Droplet on a Textured Surface

Surfaces covered with hydrophobic micro-/nanoscale textures can allow water droplets to slide easily because of low contact angle hysteresis. In contrast to the case of a droplet sliding on a smooth surface, when a droplet slides on a textured surface, it must recede from the textures at its rear edge and the resultant depinning events induce a capillary wave on the surface of the droplet. Although this depinning-induced capillary wave can be observed to some extent through high-speed imaging, important parameters of the wave, such as the wavelength and frequency, and the factors that determine these parameters are not fully understood. We report direct measurements of this depinning-induced capillary wave using microelectromechanical systems (MEMS)-based force sensors fabricated on a textured surface. Such sensor measurements reveal the frequency of the vibration occurring on the surface of the droplet, from which it is possible to calculate the wavelength of the capillary wave. We show that the frequency and wavelength of the depinning-induced capillary wave during the sliding of a water droplet on a micropillar array depend upon neither the size of the droplet nor its sliding velocity. However, the frequency (wavelength) decreases (increases) as the pitch of the micropillar array increases. We argue that the wavelength of the depinning-induced capillary wave is equal to the maximum length of the liquid bridges that develop at the micropillars before depinning. This hypothesis is confirmed by comparing the wavelengths obtained from the sensor measurements to the maximum liquid-bridge lengths calculated from observations using a high-speed camera.

MEMS-based pressure sensor with a superoleophobic membrane for measuring droplet vibration

We report on a sensor design to measure the vibration of small droplets. The sensor consists of a piezoresistive cantilever and a chamber covered with a superoleophobic membrane. The vibration of a droplet on the membrane causes the pressure of the chamber to change. Since the cantilever is able to detect a pressure change of less than 0.1 Pa, the vibration of the droplet can be precisely measured by the cantilever. In comparison to previously developed MEMS-based force sensor to measure the droplet vibration, the current sensor design offers several benefits including: wide range of usable liquids, simple sensing scheme (only one sensor is required) and capability to be disposable. With these advantages, our method is believed to be useful in measuring viscosity of small droplet for point-of-care application.

Mems force sensor array for evaluating the contractility of IPS cell-derived cardiomyocytes

We report a force sensor array that can measure iPS cell-derived cardiomyocytes (iPS-CMs) contractility with high sensitivity and high temporal resolution. The fabricated device has six piezoresistive cantilevers whose sensitivities were higher than 9.1 × 10−5 nN−1. Using the device, we measured the contraction force of an iPS-CMs layer. As a result, contractile forces ranging from 1 to 12 nN were measured.

Cantilever array for measuring traction forces of cells in a confined space

This paper reports a sensor to directly measure the traction forces generated by migrating cells in a confined space. The sensor consists of an array of miniaturized piezoresistive cantilevers which are surrounded by a pluronic-F127 pattern. The pattern allows the self-alignment and directional migration of the cells to the cantilever array as the adhesion of cells to pluronic-F127 is poor. Our method enables the quantitative evaluation of the traction forces of cells with controlled migrating direction.