Patterning and Reversible Actuation of Liquid Gallium Alloys by Preventing Adhesion on Rough Surfaces

Abstract

This work reports a simple approach to form, study, and utilize rough coatings that prevent the adhesion of gallium-based liquid metal alloys. Typically, liquids with large interfacial tension do not wet nonreactive surfaces, regardless of surface topography. However, these alloys form a surface oxide “skin” that adheres to many substrates, even those with low surface energy. This work reports a simple approach to render closed channels and surfaces, including soft materials, to be “oxide-phobic” via spray-coating (NeverWet, which is commercially available and inexpensive). Surface spectroscopic techniques and metrology tools elucidate the coatings to comprise silica nanoparticles grafted with silicones that exhibit dual length scales of roughness. Although prior work shows the importance of surface roughness in preventing adhesion, the present work confirms that both hydrophobic and hydrophilic rough surfaces prevent oxide adhesion. Furthermore, the coating enables reversible actuation through submillimeter closed channels to form a reconfigurable antenna in the gigahertz range without the need for corrosive acids or bases that remove the oxide. In addition, the coating enables open surface patterning of conductive traces of liquid metal. This shows it is possible to actuate liquid metals in air without leaving neither metal nor oxide residue on surfaces to enable reconfigurable electronics, microfluidics, and soft electrodes.

Liquid Metal–Based Soft Microfluidics

“…Such nonstick and corrosion resistance coating can be applied for a variety of applications, ranging from electrodes (conductive coating) to reconfigurable antennas (nonconductive coating). For example, the rough coating of NeverWet on PDMS was employed for the reversible actuation of liquid metal inside a closed channel, which was employed for an antenna device with adjustable behavior (adjustable operating frequency) …”

“…d) Liquid metal reversible actuation in a NeverWet coated microchannel. Reproduced with permission . Copyright 2018, American Chemical Society.…”

“…LM macrosized droplets, similar to water droplets, tend to achieve a spherical shape in air due to the tendency to minimize the surface energy. The wetting and dewetting of LM can be tuned by the adjustment of the surface energy and surface roughness of the wetted substrate . However, distinguished from a water droplet, the LM shows some unique features in both bulk and at its surface.…”

Shear‐Driven Direct‐Write Printing of Room‐Temperature Gallium‐Based Liquid Metal Alloys

“…[47] In addition, although the role of mechanical properties of the substrate on adhesion is not explicitly studied, there are no obvious trends from the data in Figure 2c when comparing adhesion to rigid versus soft substrates. Interestingly, even though Teflon failed the "peck test," our recent studies found that EGaIn can pin to perfluorinated surfaces by advancing the metal against the surface.…”

“…Retracting the nozzle reveals if the metal adheres. [46][47][48] Various treatments (deionized water rinsing, UVO exposure for 20 min, silanization, and Soxhlet extraction) are used to modify the surface chemistry and cleanliness. Figure 2a is representative of a "pass" and Figure 2b is representative of a "fail;" i.e., essentially no metal adheres to the substrate.…”