Giant and switchable surface activity of liquid metal via surface oxidation

Mohammad Rashed Khan; Collin B. Eaker; Edmond F. Bowden; Michael D. Dickey

doi:10.1073/pnas.1412227111

New Research In

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved August 21, 2014 (received for review June 30, 2014)

Significance

We present a method to control the interfacial energy of a liquid metal via electrochemical deposition (or removal) of an oxide layer on its surface. Unlike conventional surfactants, this approach can tune the interfacial tension of a metal significantly (from ∼7× that of water to near zero), rapidly, and reversibly using only modest voltages. These properties can be harnessed to induce previously unidentified electrohydrodynamic phenomena for manipulating liquid metal alloys based on gallium, which may enable shape-reconfigurable metallic components in electronic, electromagnetic, and microfluidic devices without the use of toxic mercury. The results also suggest that oxides—which are ubiquitous on most metals and semiconductors—may be harnessed to lower interfacial energy between dissimilar materials.

Abstract

We present a method to control the interfacial tension of a liquid alloy of gallium via electrochemical deposition (or removal) of the oxide layer on its surface. In sharp contrast with conventional surfactants, this method provides unprecedented lowering of surface tension (∼500 mJ/m² to near zero) using very low voltage, and the change is completely reversible. This dramatic change in the interfacial tension enables a variety of electrohydrodynamic phenomena. The ability to manipulate the interfacial properties of the metal promises rich opportunities in shape-reconfigurable metallic components in electronic, electromagnetic, and microfluidic devices without the use of toxic mercury. This work suggests that the wetting properties of surface oxides—which are ubiquitous on most metals and semiconductors—are intrinsic “surfactants.” The inherent asymmetric nature of the surface coupled with the ability to actively manipulate its energetics is expected to have important applications in electrohydrodynamics, composites, and melt processing of oxide-forming materials.

Footnotes

↵¹M.R.K. and C.B.E. contributed equally to this work.
↵²To whom correspondence should be addressed. Email: mddickey{at}ncsu.edu.

Author contributions: M.R.K., C.B.E., E.F.B., and M.D.D. designed research; M.R.K. and C.B.E. performed research; M.R.K., C.B.E., E.F.B., and M.D.D. analyzed data; and M.R.K., C.B.E., and M.D.D. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1412227111/-/DCSupplemental.