Sessile droplet de-pinning: New life for gravimetric data

Using three different types of surfaces as exemplars, we report a gravimetric method as a viable tool for studying the de-pinning process. Namely, the de-pin time, τdτd (the time required for a horizontal sessile droplet to de-pin at the triple phase line on a given substrate), is estimated without using a time consuming and expensive video imaging system. This is made possible by deciphering the non-linear portion of mass vs time data of an evaporating sessile droplet. Typical gravimetric glass-substrate evaporative mass loss vs time data has two regimes: a long, linear regime followed by a short, non-linear regime. Traditionally, researchers extract only the evaporation rate of a droplet from the linear regime but discard (by truncating the data) or ignore (thus deriving no information from) the non-linear regime. The origin of the linear to non-linear transition, found almost universally in gravimetric data, persists unremarked upon. By constructing three very different types of surfaces and comparing gravimetric data with video imaging data taken simultaneously, we report the transition is correlated to the onset of the de-pinning event in each case. This realization enables us to measure the de-pin time, τdτd, with gravimetric data only; i.e., without the video system, gathering more information from gravimetric data than previously considered. The method has application in estimating the de-pin time of a droplet deposited on a substrate that yields poor top-view contrast for videography, such as a water droplets on silicon wafers or glass substrates. Finally, gravimetric data is more accurate for evaporation modeling when substrate/droplet interaction areas are not circular.

By plotting mass-loss due to evaporation and area exposed to a substrate against time, we discover the beginning of the sessile droplet de-pinning event corresponds to the time at which the evaporative mass-loss curve deviates from linear. We use this to predict de-pinning events using only an analytical balance and a microprocessor.Figure optionsDownload as PowerPoint slide