Liquid metal enabled microfluidics

Abstract

Several gallium-based liquid metal alloys are liquid at room temperature. As 'liquid', such alloys have a low viscosity and a high surface tension while as 'metal', they have high thermal and electrical conductivities, similar to mercury. However, unlike mercury, these liquid metal alloys have low toxicity and a negligible vapor pressure, rendering them much safer. In comparison to mercury, the distinguishing feature of these alloys is the rapid formation of a self-limiting atomically thin layer of gallium oxide over their surface when exposed to oxygen. This oxide layer changes many physical and chemical properties of gallium alloys, including their interfacial and rheological properties, which can be employed and modulated for various applications in microfluidics. Injecting liquid metal into microfluidic structures has been extensively used to pattern and encapsulate highly deformable and reconfigurable electronic devices including electrodes, sensors, antennas, and interconnects. Likewise, the unique features of liquid metals have been employed for fabricating miniaturized microfluidic components including pumps, valves, heaters, and electrodes. In this review, we discuss liquid metal enabled microfluidic components, and highlight their desirable attributes including simple fabrication, facile integration, stretchability, reconfigurability, and low power consumption, with promising applications for highly integrated microfluidic systems.

Recent progress on liquid metals and their applications

“…Chemical passivation is an intrinsic property of Ga that enables Ga and Ga-based alloys to be used as dielectrics, although it also hinders their general use as liquid conductors. The surfaces of pure Ga or eutectic Ga-based alloys are easily oxidised and form an amorphous Ga oxide layer in the ambient environment that decreases the surface tension of the liquid metal [11][12][13]. The thickness of the intrinsically formed Ga oxide surface layer is 0.5-3 nm [14][15][16].…”

“…There are now several excellent reviews covering applications of liquid metal on different specific properties [1,8,11,15,16]. Here, we will cover the current progress on the main advantaged properties of Ga-based liquid metal and the applications derived from these advantages.…”

“…This method is an excellent way to generate spatial temperature gradients by altering the rotational velocity and configuration of vortices, which can be controlled by changing the frequency and polarity. This approach replaced the method of using acousto thermal [11,57] or microwave-induced heating, it means that the system does not require a lot of hot spots [13] or localised hot spots for heating in the liquid. These pumps can generate more complex temperature profiles and gradients [59].…”

Gallium-Based Room-Temperature Liquid Metals: Actuation and Manipulation of Droplets and Flows

“…Droplet generation has been studied extensively in the field of microfluidics (Xu and Attinger, 2009;Yu et al, 2011;Khoshmanesh et al, 2017;Zhu and Wang, 2017). Various existing methods are applicable to liquid metal including manual methods (Yu et al, 2014), flow focusing via microchannels (Thelen et al, 2012), molding (Mohammed et al, 2014), and inkjet printing (Li et al, 2015(Li et al, , 2016.…”

“…EGaIn is a similar eutectic composition of 75.5%wt of gallium and 24.5%wt of indium (Dickey et al, 2008). Demonstrating superior performances in various aspects, gallium-based liquid metals have been explored for many novel applications, such as microfluidics devices (Khoshmanesh et al, 2017), stretchable electronics (Wang et al, 2015c;Bartlett et al, 2016), reconfigurable devices (Wang et al, 2015b), electronics cooling (Ma and Liu, 2007), vacuum pumping (Tang et al, 2015a), and painted conductive electrodes in liquid droplet actuation (Eaker et al, 2017). However, there are also some challenges in working with these liquid metals.…”