Paradoxical Long-Timespan Opening of the Hole in Self-Supported Water Films of Nanometer Thickness Z. Barkay* ,† and E. Bormashenko ‡ † Wolfson Applied Materials Research Center, Tel Aviv University, Ramat-Aviv 69978, Israel ‡ Engineering Faculty, Chemical and Biotechnological Engineering Department, Ariel University, Ariel 40700, Israel *S Supporting Information ABSTRACT: The opening of holes in self-supported thin (nanoscaled) water films has been investigated in situ with the environmental scanning electron microscope. The opening of a hole occurs within a two-stage process. In the first stage, the rim surrounding a hole is formed, resembling the process that is observed under the puncturing of soap bubbles. In the second stage, the exponential growth of the hole is observed, with a characteristic time of a dozen seconds. We explain the exponential kinetics of hole growth by the balance between inertia (gravity) and viscous dissipation. The kinetics of opening a microscaled hole is governed by the processes taking place in the nanothick bulk of the self-supported liquid film. Nanoparticles provide markers for the visualization of the processes occurring in self-supported thin nanoscale liquid films. ■ INTRODUCTION The dynamics of the opening of holes (also named voids or pores) in liquid films was subjected to intensive theoretical and experimental research 1−9 mainly because of the interest in the “life and death of bubbles”, as was worded by De Gennes et al. in ref 4. Two very di fferent physical situations were investigated, namely, the opening of holes in water films containing surfactants 1−3 and the opening of holes formed in viscous liquid films, such as silicone oil, molten glass, and polymer solutions. 4−6 Two main mechanisms of hole opening were distinguished, the first of which is the capillarity/inertia opening of a hole. 1,2 Within this mechanism, after puncturing a film the liquid is collected in a rim expanding at a constant velocity. The kinetics results from a balance between the surface energy and the kinetic energy of the rim, and it is inherent to the bursting of soap bubbles. 1,2 The formation of rims was also observed in the initial stage of the growth of holes in viscous films. 6 In contrast, the exponential growth of holes was observed for viscous liquid films. 4,5 The debatable effects due to the line tension, as expected for nanoscaled holes, have been discussed. 7 In this article, we focus on the in situ study of the hole opening in self-supported water films using environ- mental scanning electron microscopy (ESEM). 10 Electron microscopy imaging methods for the dynamic study of liquids mainly involve ESEM and transmission electron microscopy (TEM) combined with silicon nitride and graphene liquid cells. 11,12 In situ TEM research with a liquid cell was performed 13,14 for gold nanoparticles (NPs) encapsulated in liquid nanodrops. Nanovoid generation in the films was attributed to a radiology effect, and the dynamics was explored 15 for thin (10 nm) liquid films over silicon nitride substrate in a TEM liquid cell. The noncircular shape of the voids was attributed to substrate pinning. In situ imaging in ESEM by transmitted electrons provides an alternative approach that uses a wet-scanning transmission electron microscope (wet-STEM) 16,17 detector. The wet-STEM method has been applied for the nanoscale imaging of thin films under environmental conditions with better resolution (of a few nanometers) than the ESEM of bulk samples. Wet-STEM studies 18−22 were performed for self-supported liquid films showing spontaneous film rupture under filmwise condensa- tion. The initiation of hole formation under an e-beam was attributed to radiology 23−25 effects. e-beam heating was mentioned 26 for the self-assembly process of surfactant-treated NPs with the observation of delayed film rupture on the time scale of seconds. In this article, we quantify the hole-opening phenomenon for finite-sized self-supported nanometer-thick liquid films. Differ- ent characteristic time length scales refer to spontaneous hole opening and delayed hole opening. In particular, we refer to a two-stage process of spontaneous opening followed by delayed opening. It is shown here that the quantitative exponential delayed growth of the hole may be reasonably related to the balance of gravity and viscous dissipation. ■ EXPERIMENTAL DETAILS An investigation was carried out using a wet-STEM detector in the FEI Quanta 200 field emission gun (FEG) ESEM. The FEI wet-mode Received: March 14, 2017 Revised: April 23, 2017 Published: April 25, 2017 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.7b00861 Langmuir XXXX, XXX, XXX−XXX