The 5th Japan-China-Korea MEMS/NEMS with NANO KOREA 2014
A novel microfluidic platform for recovery of non-wetting Newtonian
characteristics of GALINSTAN® using gas permeable PDMS membrane
Gil-dong Hong1, Dong-Weon Lee1*
1
School of Advanced Materials Science and Engineering, Saaa University,
City Zipcode, Country
Galinstan®, a non-toxic metal eutectic alloy, is in liquid phase at room temperature and has been investigated for
various applications including biology [1], RF micro switch [2] and tunable frequency selective surface (FSS) [3].
However, its surface is readily oxidized in air environment and forms a thin oxide layer causing the alloy to adhere to
almost any surface [4]. It was reported that Galinstan ® behaves like true liquid in the below 1 ppm of oxygen
environment [5], but this requires good hermetic packaging for microfluidic platform for Galinstan®. Microfluidic
channels filled with diluted hydrochloric acid (HCl) showed removal of oxide skin in eutectic GaIn alloy [6], but such
HCl filled microfluidic channel may not be applicable for most applications. We reported a super-lyophobic PDMS
micro-tunnel using hierarchical micro/nano surface textured inner walls to manipulate “oxidized” Galinstan ® droplet [7].
Although there have been several efforts [5-7], up to this point there is no simple, yet universally applicable microfluidic
platform technology that maintains non-wetting Newtonian fluid characteristics of Galinstan®. In this paper, we report a
novel microfluidic platform using gas permeable polydimethylsiloxane (PDMS) membrane and integrated HCl reservoir
to constantly maintain non-wetting Newtonian fluid characteristics of Galinstan® in the microfluidic channel.
We found that a simple technique using PDMS membrane on top of the HCl reservoir allows permeation of HCl
vapor and transforms oxidized viscoelastic phase Galinstan® into non-wetting Newtonian true liquid phase Galinstan®
(Fig.1). Various thicknesses (0.25, 0.5, 1 and 2 mm) of PDMS membranes and various concentrations of HCl (37, 30 and
25 wt%) solutions in air environment were tested. Fig. 2 shows 7.8 μL Galinstan® droplet’s contact angle changes over
time for different PDMS membrane thickness. Fig. 3 shows Galinstan ® droplet’s contact angle changes over time for
different HCl concentrations. As expected, thinner PDMS membrane and higher HCl concentration solutions allow
higher contact angle changes. It was found that HCl solution higher than 30 wt% with < 1mm PDMS membrane make
the Galinstan® droplet’s contact angle increased and saturated within 100 seconds and allows recovery of its Newtonian
fluid characteristics.
We fabricated a microfluidic channel with a ‘T’-junction with integrated HCl reservoir all in replicated PDMS using
SU-8 mold (Fig. 4). The device comprised a straight 500 μm wide channel, a 250 μm wide serpentine shape control
channel, and 3 ports to inject Galinstan® and apply air pressure. The PDMS microfluidic channel and the PDMS HCl
reservoir were bonded with another PDMS membrane (500 µm thick) in between (Fig 4g). Fig 5a shows top view of the
fabricated platform for Galinstan® microfluidics. As pointed, Galinstan® oxidizes instantly as it is exposed to air. Fig. 5b
shows oxidized Galinstan® in the microfluidic channel which clearly shows its viscoelastic characteristics of oxidized
Galinstan® which wetted the inner wall surface of the PDMS microfluidic channel. However, Galinstan ® in this
microfluidic channel quickly recovers its non-wetting true liquid phase Newtonian fluid characteristics due to HCl vapor
diffusion (Fig. 4h and Fig. 5c) and subsequent reaction with oxidized Galinstan®. We also successfully demonstrated
generation, merge and separation of Galinstan® slugs using this novel microfluidic platform. Fig. 6a shows a series of still
images taken from a video showing a method of generation of Galinstan® slug with on-demand control of air pressure.
Fig. 6b shows multiple Galinstan® slugs generated in a channel.
We believe that this novel microfluidic platform for recovery of non-wetting Newtonian true liquid phase Galinstan®
may unleash full potentials of a wide variety of liquid metal based applications.
※ Text Page Limit: 1 page with single space
*Corresponding Author: Tel. +82-(31)-200-xxxx, E-mail.: abcd@efg.edu
References
[1] M. Knoblauch et al., Nature Biotechnology, 17, 906, 1999.
[2] P. Sen, C.-J. Kim, J. MEMS, 18 (5), 990, 2009.
[3] M. Li, B. Yu, N. Behdad, IEEE Microwave and Wireless Component Letters, 20 (8), 423, 2010.
[4] F. Scharmann et al., Surf. Interface Anal., 36, 981, 2004.
[5] T. Liu, P. Sen, C.-J. Kim, IEEE MEMS Conference, 560, 2010.
[6] M. Dickey et al., Adv. Functional Materials, 18, 1097, 2008.
[7] D. Kim, D-W Lee, W. Choi, JB Lee, IEEE MEMS Conference, 1005, 2012.
The 5th Japan-China-Korea MEMS/NEMS with NANO KOREA 2014
Oxidized
Galinstan® droplet
True liquid phase
Galinstan® droplet
Si
Microfluidic channel
SU-8
Thin PDMS membrane
(0.25, 0.5, 1, 2 mm)
Figure 1: Oxidized Galinstan® droplet turned into true
liquid phase Galinstan® droplet by chemical reaction
with HCl vapor diffused through the PDMS membrane.
Oxidized
PDMS
5 µL HCl solution
(25, 30, 37 wt%)
Si
HCl reservoir
PDMS + PDMS
bonding
True liquid phase
Galinstan®
in channel
HCl reservoir integrated
microfluidic channel
※ Image Page Limit: 1Figure
page
with single
4: Fabrication
sequence of space
the HCl reservoir integrated
microfluidic platform for Galinstan®: (a),(d) SU-8 mold, (b),(e)
PDMS coating, (c),(f) replicated PDMS, (g) PDMS-PDMS
bonding, (h) HCl vapor diffusion through the PDMS membrane.
Figure 2: Galinstan® droplet contact angles as a function
of diffusion time of 37 wt% HCl for various PDMS
membrane thicknesses.
True liquid phase for
30 and 37 wt% HCl
‘Dewetting’
Oxidized even after 1000s
diffusion for 25 wt% HCl
Figure 3: HCl concentration dependency of Galinstan®
droplet contact angles for various HCl concentrations.
Oxidized
HCl vapor diffusion
for 100 seconds
True
liquid
phase
Figure 5: (a) Top view of the HCl reservoir integrated
microfluidic platform , (b) oxidized Galinstan® in the channel,
and (c) true liquid phase Galinstan® after HCl vapor diffusion.
(a)
Generated Galinstan® slugs
(b)
Figure 6: (a) A series of still images taken from a real-time video of generation of a Galinstan® slug by on-demand control air
pressure through three ports, (b) generated multiple Galinstan® slugs.