Drops That Change Their Mind: Spontaneous Reversal from Spreading to Retraction

Abstract

A liquid drop may spread faster on surfaces when surfactants are added. Here we show that after some time the spreading in such systems can, under certain conditions, spontaneously reverse to retraction and the droplet pulls itself back, receding from areas it has just recently wetted, elevating its center of mass in a jerklike motion. The duration from drop placement to the onset of retraction ranges from hours to less than a second primarily as a function of surfactant concentration. When the retraction is asymmetric, it results in drop motion, and when it is symmetric, the mass of the drop collects itself on its spot. This phenomenon, which was predicted theoretically in 2014, is apparently a general one for drops with surfactants; however, other factors, such as evaporation and contamination, prevented its observance so far.

Evaporation of ethanol/water mixture droplets on micro-patterned PDMS surfaces

“…For mixture droplets with the initial ethanol concentration of 75% on the pillar-arrayed PDMS surfaces with the pillar-to-pillar spacing of 20 or 50 lm, the contact angle begins to increase at the end of the first stage-CCR stage and increases to a maximum. This phenomenon is similar to that observed by Tadmor [57,58]. Due to the high volatility of ethanol, there will be a greater increase in liquid-vapor interfacial tension as well as a slight increase in solid-liquid interfacial tension (as shown in Fig.…”

“…At first, the contact angle decreases with time, then it continuously increases until it reached to the maximum and keeps nearly unchanged for a period (sometimes no CCA stage is observed) and finally it decreases con-tinuously. The stage at which the contact angle continues to increase can also be explained by using Tadmor's theory [57,58]. Fig.…”

Wetting and coalescence: beyond single-phase flows

“…Contrary to many previous works on autophobing [141][142][143][144][145][146][147], we do not 4.5. Effect of the molecular structure see an initial spreading phase followed by a retraction to the quasi-steady θ (see Fig.…”

Magnetic field effects on surfactants adsorption on the solid surface as regards of its wettability

“…Nowadays it is hard to imagine our life without synthetic surfactants in both everyday life and industry. Adsorption of surfactants and their mixtures at the liquid-gas, liquid-liquid and solid-liquid interfaces can change their interfacial properties significantly, therefore they find many practical applications such as in mineral and petroleum processing, biological systems, health and personal care products, foods, crop protection, corrosion inhibition and others (Schramm et al, 2003;Paria and Khilar, 2004;Zhang and Somasundaran, 2006) as well as scientific challenges such as spreading reversal to retraction (Tadmor et al, 2019;Tadmor et al, 2020;Tadmor et al, 2021). One of the important applications of surfactants are enriching processes of the mineral ore and among them the flotation process where the used surfactants are called 'collectors'.…”