Evaporation dynamics of a sessile droplet on glass surfaces with fluoropolymer coatings: focusing on the final stage of thin droplet evaporation

Abstract

The evaporation dynamics of a water droplet with an initial volume of 2 μl from glass surfaces with fluoropolymer coatings are investigated using the shadow technique and an optical microscope. The droplet profile for a contact angle of less than 5° is constructed using an image-analyzing interference technique, and evaporation dynamics are investigated at the final stage. We coated the glass slides with a thin film of a fluoropolymer by the hot-wire chemical vapor deposition method at different deposition modes depending on the deposition pressure and the temperature of the activating wire. The resulting surfaces have different structures affecting the wetting properties. Droplet evaporation from a constant contact radius mode in the early stage of evaporation was found followed by the mode where both contact angle and contact radius simultaneously vary in time (final stage) regardless of wettability of the coated surfaces. We found that depinning occurs at small contact angles of 2.2-4.7° for all samples, which are smaller than the measured receding contact angles. This is explained by imbibition of the liquid into the developed surface of the "soft" coating that leads to formation of thin droplets completely wetting the surface. The final stage, which is little discussed in the literature, is also recorded. We have singled out a substage where the contact line velocity is abruptly increasing for all coated and uncoated surfaces. The critical droplet height corresponding to the transition to this substage is about 2 μm with R/h = 107. The duration of this substage is the same for all coated and uncoated surfaces. Droplets observed at this substage for all the tested surfaces are axisymmetric. The specific evaporation rate clearly demonstrates an abrupt increase at the final substage of the droplet evaporation. The classical R law is justified for the complete wetting situation where the droplet is disappearing in an axisymmetric manner.

Influence of ambient conditions on evaporation and transport of respiratory droplets in indoor environment

“…We are not including dynamic situations in case for evaporation investigation because later on we will see that the larger droplets will settle on the ground prior to evaporation. After settlement on ground, diameter reduction is a complex phenomenon as droplet evaporation dynamics depend on the ground surface [ [54] , [55] , [56] , [57] , [58] , [59] , [60] , [61] , [62] , [63] ]. The influence of ambient temperature and relative humidity has been studied for 10 μm and 50 μm droplets.…”

Gravitational Effect in Evaporating Binary Microdroplets

“…We obtain different Ar, larger or smaller than a unit by varying a series of droplet heights, which corresponds to cases with or without gravitational effects, respectively. Finally, we simulate the process numerically, finding good agreement with the experimental results and again confirming our interpretation.The evaporation of a microdroplet on a flat substrate has attracted a lot of attention because of its beautiful and phenomenologically rich fluid dynamics [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and its relevance in various technological applications, such as medical diagnostics [15] and the fabrication of electronic devices [16]. For many of these applications, an understanding of the internal flow structure is crucial.…”

CLeANFIT – Contact‐Less Axial Nearfield‐Based Fluorescence Imaging Topography: A Method for 3D Micro‐ and Nanotopography Characterization

“…Recent studies dedicated to the final droplet drying stage dynamics, [ 64 ] divided the final stage into several substages, where increasing contact line velocities are also observed. [ 63 ] However, the resolution of the presented data recorded by interference imaging limited the observable thicknesses to 300 nm, in a study where 2 µL water droplets were deposited on a fluopolymer‐coated glass substrate. Using CLeANFIT, the contact line transition could be observed for a 3 µl droplet of ATTO488‐doped water/PVA with an axial resolution reaching down to tens of nanometers, which far exceeds conventional droplet shape analyzer tools and even more recent optical imaging techniques.…”

“…[ 66–70 ] Targeting nanoinkjet printing, various characterization techniques have been proposed for the observation and analysis of the involved nano‐ to picoliter droplets. For morphological studies, both in the dried and the liquid states, confocal microscopy, [ 71 ] SEM, [ 72 ] AFM, [ 70 ] contact profilometry, [ 73 ] and interferometry [ 63 ] have been tested. However, all these methods are either limited by their axial resolution, or by damages on the material surface due to direct contact of the sample with a tip, preventing in situ analysis.…”

“…Understanding in detail the drying and contact line dynamics provides relevant insights towards drop-on-demand applications [62] with specific evaporation rates. [63] Thus, we perform single line scanning across the center of a drying droplet with higher lateral and temporal resolution. Figure 6a shows the FLIM scans taken between 17 and 22 min after deposition, with time interval of 20 s per frame.…”