PDMS based coplanar microfluidic channels for the surface reduction of oxidized Galinstan

Abstract

Galinstan has the potential to replace mercury - one of the most popular liquid metals. However, the easy oxidation of Galinstan restricts wide applicability of the material. In this paper, we report an effective reduction method for the oxidized Galinstan using gas permeable PDMS (polydimethlysiloxane)-based microfluidic channel. The complete study is divided into two parts - reduction of Galinstan oxide and behavior of reduced Galinstan oxide in a microfluidic channel. The reduction of Galinstan oxide is discussed on the basis of static as well as dynamic angles. The contact angle analyses help to find the extent of reduction by wetting characteristics of the oxide, to optimize PDMS thickness and to select suitable hydrochloric acid (HCl) concentration. The highest advancing angle of 155° and receding angle of 136° is achieved with 200 μm thick PDMS film and 37 wt% (weight percent) HCl solution. The behavior of reduced Galinstan oxide is analyzed in PDMS-based coplanar microfluidic channels fabricated using a simple micromolding technique. Galinstan in the microfluidic channel is surrounded by another coplanar channel filled with HCl solution. Due to the excellent permeability of PDMS, HCl permeates through the PDMS wall between the two channels (interchannel PDMS wall) and achieves a continuous chemical reaction with oxidized Galinstan. A Lab VIEW controlled syringe pump is used for observing the behavior of HCl treated Galinstan in the microfluidic channel. Further optimization of the microfluidic device has been conducted to minimize the reoxidation of reduced Galinstan oxide in the microfluidic channel.

Hydrochloric acid-impregnated paper for gallium-based liquid metal microfluidics

“…There have been several efforts to overcome this issue by using non-wettable surface against oxidized gallium-based liquid metal. It was reported that HCl solution or vapor can chemically remove the oxide skin from gallium-based liquid metal [19][20][21]. Rather than removing oxide skin, it was also reported that hierarchical micro-/nano-scale structured surface is super-lyophobic and oxidized Galinstan ® does not adhere to the engineered surface [22].…”

Real-time dynamically reconfigurable liquid metal based photolithography

“…It is known that gallium-based liquid metal have a challenging drawback that it gets easily oxidized and wets on almost any surfaces. However, once the liquid metal is exposed to HCl vapor, the oxide layer is removed so that liquid metal can recover non-wetting characteristic and thus it can be controllable [6,7]. In this work, to reconfigure liquid metal pattern in the microfluidic channels, the 7-segments microfluidic channel is placed on top of a hydrochloric acid (HCl) reservoir, where HCl solution is stored, to have chemical reaction with HCl vapor through gas permeable PDMS and recover non-wetting characteristic of liquid metal.…”

Selectively plated stretchable liquid metal wires for transparent electronics

“…In our previous studies, the wetting behaviors of oxidized or HCl-treated Galinstan droplets on PDMS substrates are described [38,40]. Reduced Galinstan exhibits superlyophobicity on a PDMS substrate, as shown in Fig.…”

“…S4(c)). It should be noted that an HCl solution concentration less than 16 wt% cannot completely remove the oxidation layer of Galinstan at room temperature [38]. To minimize corrosion of the adhesion layer (Cr) and ensure the uniformity of the pattern edge, a lower HCl solution concentration (16 wt%) is employed in our experiments.…”

“…S1) in details. The reduced liquid-metal droplet has different behaviors on metal and non-metal substrates [38]. The current-voltage (I-V) characteristics of a stretchable circuit integrated with a light-emitting diode (LED) are obtained under various conditions such as bending, twisting, and stretching.…”