Are Contact Angle Measurements Useful for Oxide-Coated Liquid Metals?

Abstract

This work establishes that static contact angles for gallium-based liquid metals have no utility despite the continued and common use of such angles in the literature. In the presence of oxygen, these metals rapidly form a thin (∼1−3 nm) surface oxide "skin" that adheres to many surfaces and mechanically impedes its flow. This property is problematic for contact angle measurements, which presume the ability of liquids to flow freely to adopt shapes that minimize the interfacial energy. We show here that advancing angles for a metal are always high (>140°)even on substrates to which it adheresbecause the solid native oxide must rupture in tension to advance the contact line. The advancing angle for the metal depends subtly on the substrate surface chemistry but does not vary strongly with hydrophobicity of the substrate. During receding measurements, the metal droplet initially sags as the liquid withdraws from the "sac" formed by the skin and thus the contact area with the substrate initially increases despite its volumetric recession. The oxide pins at the perimeter of the deflated "sac" on all the surfaces are tested, except for certain rough surfaces. With additional withdrawal of the liquid metal, the pinned angle gets smaller until eventually the oxide "sac" collapses. Thus, static contact angles can be manipulated mechanically from 0°to >140°due to hysteresis and are therefore uninformative. We also provide recommendations and best practices for wetting experiments, which may find use in applications that use these alloys such as soft electronics, composites, and microfluidics.

Line Tension and Drop Size Dependence of Contact Angle at the Nanoscale

“…To address these issues, we have previously presented the use of a liquid metal as a liquid with a barely measurable evaporation rate [22] and demonstrated robotically assisted contact angle measurements in a scanning electron microscope (SEM) [23]. To address the issue of oxide layer growth on the liquid metal droplets, which would prevent them from being useful for contact angle measurements [24], our experiments take place in the low-oxygen vacuum atmosphere of an SEM chamber.…”

Electrically Induced Liquid Metal Droplet Bouncing

“…NaOH (1M) was used as an electrolyte to create a basic pH environment that dissolves the surface oxide of the liquid metal, 37 which would otherwise interfere with the wetting dynamics and contact angle. 38 The dissolution of gallium oxide proceeds through the following reaction 39 Ga O 2NaOH 3H O 2Na Ga(OH) Resonant Frequency. The resonant frequency (ω) of a freely oscillating liquid droplet for the nth mode oscillation is given as 41−43 ω 2 = (σ LV n(n − 1)(n + 2)/ρ)q 3 , where σ LV is the interfacial tension of the liquid metal and surrounding electrolyte interface, n is the mode number, ρ is the density of EGaIn, and q is the characteristic length of the droplet.…”

Liquid Metal Patterned Stretchable and Soft Capacitive Sensor with Enhanced Dielectric Property Enabled by Graphite Nanofiber Fillers

“…Prior to electrode printing, the contact angles made by water and EGaIn on the composite (PGNC@6wt %) were compared to that of pure PDMS and no significant difference was observed (Figure 7a-d) which indicated negligible changes in surface wetting behaviors by the GNF fillers. Receding the EGaIn droplets left behind traces of oxide (Figure 7e-h) which signifies oxide-substrate adhesion possibly by van der Waals interaction [89]. Using the camera of the contact angle goniometer, tilting of a tiny droplet of pure EGaIn on composite was recorded (Video S1) and it was observed that the droplet adhered to the composite even when the tilt angle was 90 • , signifying resistive adhesive force of the droplet's oxide layer to the composite's surface against gravitational force [38].…”