Letter pubs.acs.org/JPCL
Nanoscale View of Dewetting and Coating on Partially Wetted Solids Yajun Deng, Lei Chen, Qiao Liu, Jiapeng Yu, and Hao Wang* Laboratory of Heat and Mass Transport at Micro-Nano Scale, College of Engineering, Peking University, and Key Lab of Theory and Technology for Advanced Batteries Materials, Beijing 100871, China S Supporting Information *
ABSTRACT: There remain significant gaps in our ability to predict dewetting and wetting despite the extensive study over the past century. An important reason is the absence of nanoscopic knowledge about the processes near the moving contact line. This experimental study for the first time obtained the liquid morphology within 10 nm of the contact line, which was receding at low speed (U < 50 nm/s). The results put an end to longstanding debate about the microscopic contact angle, which turned out to be varying with the speed as opposed to the constant-angle assumption that has been frequently employed in modeling. Moreover, a residual film of nanometer thickness ubiquitously remained on the solid after the receding contact line passed. This microscopic residual film modified the solid surface and thus made dewetting far from a simple reverse of wetting. A complete scenario for dewetting and coating is provided.
I
(ESEM).25−28 Nano droplets movement has been examined through transmission electron microscopy (TEM), which included a microfabricated liquid cell with electron translucent membranes.29,30 Scanning electron microscopy (SEM) can also directly measure ionic liquid, which is nonvolatile and ionic conductive.31 These electron microscopy methods are used for droplet dynamics imaging rather than accurate film thickness measurement. Remarkable developments have recently been made using tapping-mode atomic force microscopy (TM-AFM, also called intermittent contact mode AFM).32 Herminghaus et al.,32−34 Yu et al.,35 and Ma et al.36 measured profiles and contact angles for static liquids. A pioneering work on dynamic wetting by Chen et al.37 measured advancing contact lines to show that the interface of an advancing thin film followed the macroscopic profile until bending in a convex profile within 10−20 nm of the substrate. The local angle at the contact line increased with the advancing speed. This study investigated the dewetting of nonvolatile droplets on partially wetted surfaces. The experimental system was based on a state-of-the-art TM-AFM (MFP-3D-BIO, Asylum Research) that has low noise performance for high-resolution imaging of the most delicate samples like proteins or liquid surfaces. The system has shown to have excellent accuracy and stability on static35 as well as spreading droplets.37 The test section was in a chamber where the atmosphere (pure nitrogen or air) and temperature (25 °C) were well-controlled. Electrostatic level was controlled by an ionizing air blower (AEROSTAT, Simco). The receding contact line was formed by transferring a millimeter-size liquid droplet onto the solid
nterfaces play an essential role in virtually all materials and applications such as coating, self-assembly, crystal growth, friction, self-cleaning, anti-icing, condensation, aerosol, and various biological processes.1−10 A refinement of our understanding of the interfacial effects is critical. Notable achievements on wettability manipulation have been made in recent years boosted by surface nanoengineering;7,11,12 however, fundamental mechanisms remain vague. There are significant gaps in our ability to model dewetting and wetting in sufficient detail to predict outcomes such as coating thickness, dynamic contact angle, and thin-film profile even for those on smooth surfaces.13 The contact-line region is the intersection of the interfaces that play central roles in the triple-phase system. It is also the least understood part due to the very small-scale and complicated intermolecular and interfacial interactions.14,15 Various hypotheses16−21 have been proposed regarding the contact-line movement mechanism, the regularization of the hydrodynamic singularity, the contact-angle hysteresis, the microscopic contact angle and its speed dependency, and the modeling approach for thin-film profile. Extensive debate has lasted for years. An important reason is that experimental validations of the various hypotheses have been less effective. The dewetting and wetting phenomena operate on scales down to molecules, while observations usually are made at resolutions of microns.22 The liquid microlayer profile of thickness 90 o) and thus had gentle interactions with the sample surface.37,38 The energy dissipated by the tip−sample interaction was calculated to be
