Ultrasmall Liquid Droplets on Solid Surfaces: Production, Imaging, and Relevance for Current Wetting Research

Abstract

The investigation of micro- and nanoscale droplets on solid surfaces offers a wide range of research opportunities both at a fundamental and an applied level. On the fundamental side, advances in the techniques for production and imaging of such ultrasmall droplets will allow wetting theories to be tested down to the nanometer scale, where they predict the significant influence of phenomena such as the contact line tension or evaporation, which can be neglected in the case of macroscopic droplets. On the applied side, these advances will pave the way for characterizing a diverse set of industrially important materials such as textile or biomedical micro- and nanofibers, powdered solids, and topographically or chemically nanopatterned surfaces, as well as micro-and nanoscale devices, with relevance in diverse industries from biomedical to petroleum engineering. Here, the basic principles of wetting at the micro- and nanoscales are presented, and the essential characteristics of the main experimental techniques available for producing and imaging these droplets are described. In addition, the main fundamental and applied results are reviewed. The most problematic aspects of studying such ultrasmall droplets, and the developments that are in progress that are thought to circumvent them in the coming years, are highlighted.

Are the Soft, Liquid-Like Structures Detected around Bacteria by Ambient Dynamic Atomic Force Microscopy Capsules?

“…In another field, it has indeed been recently discovered that certain microorganisms are taking advantage of the deliquescent nature of some minerals present in the Atacama Desert, the driest place on Earth, to survive in such a hostile environment (5). Finally, the use of deliquescent salts to stabilize and study ultrasmall liquid droplets on solid surfaces under normal ambient conditions, which is not possible with conventional liquids such as water or organic liquids due to almost instantaneous evaporation, could provide new avenues for the study of wetting properties at ultrasmall scales, a recently reviewed hot area in surface science (19), since liquid topography can be accurately tracked at high resolution using AFM.…”

“…AFM has been proposed as a high (nanometer)-resolution technique that could provide direct evidence of the presence of bacterial capsules under ambient conditions. Indeed, AFM has proved capable of scanning the surface of liquids, and this is being currently explored in areas such as the study of wetting properties at ultrasmall scales (19). In this sense, several works have recently reported the observation of capsules using ambient AFM (25,27,30,31) in the form of tiny amounts of a liquidlike substance around bacteria.…”

Formation of surface nanodroplets under controlled flow conditions

“…The understanding from this work provides valuable guidelines for producing surface nanodroplets with desired sizes by controlling the flow conditions. N anoscale droplets on a substrate (1) are an essential element for a wide range of applications, namely laboratory-on-chip devices, simple and highly efficient miniaturized reactors for concentrating products, high-throughput single-bacteria or singlebiomolecular analysis, encapsulation, and high-resolution imaging techniques, among others (2)(3)(4)(5). These droplets are of great interest also because they can have a payload and can flow internally in response to external flow.…”

In Situ Experiments in the Scanning Electron Microscope Chamber

“…A recent review by Mendez-Vilas et al (2009) has highlighted the main fundamental and applied results. Several strategies for the contact angle between water and the surface determination are reported (Stelmashenko et al, 2001;Stokes, 2001;Lau et al, 2003;Wei, 2004;Yu et al, 2006;Jung & Bhushan, 2008;Rykaczewski & Scott, 2011).…”