Spreading behavior of oil (hexadecane) droplet on flat copper surface was studied with oil having surfactants of different head groups and varying chain lengths. The droplet spreading rate decreased with increase in either surfactant [Triphenyl phosphorothionate (TPPT)] concentrations (0-2 wt%) or chain lengths (TPPT, butyl and nonly TPPT). Within an intermediate (0.01%-2 wt%) concentration range, the manner in which spreading rates decreased was different with increase in chain lengths; two spreading regimes with different spreading rates, and a cross over regimen in between. At certain surfactant concentration, the initiation of the crossover regimen occurred faster for longer chains. Such behavior was also observed with surfactants having different head groups, e.g., dodecylamine surfactants induced reduction in spreading rate spontaneously [in a few ( 10 mins) minutes for dodecanol. Such inconsistency in terms of different time frames in which spreading rate varied entailed the role of the forces (FSASS) exerted from an evolving interfacial [solid/oil (S/O) and solid/oil/air (S/O/A)] self assembled surfactant structure (SASS) over the viscous and surface tension forces. Structural evolution of SASS has been correlated to the adsorption affinity of the surfactants by virtue of its head group functionality, and arrangement including a rearrangement step. While higher (e.g., dodecylamine) adsorption affinity of surfactants enables faster SASS formation activating FSASS early on, a arrangement step correlated to an increase in FSASS ensuing pinning of the droplets, where FSASS is significant enough to counter balance the viscous/'surface tension' forces. Furthermore, it appears that there exists a rearrangement (packing) step during which the FSASS increases further leading to contraction of a droplet. SASS distributions across the S/O and S/O/A interface, if are in patches across the interfaces, created regions with different spreading powers, as FSASS is less effective in the patchy regions, and thus, leading to specific spreading behaviors such as protruding edges along the contact line, formation of dewetted regions across the S/O interface and even droplet spreading outwards with a dewetted region in the middle can also be seen.

Structurally evolving "flat solid"/oil interfacial self assembled surfactant structure steadily enforces peculiarities into oil spreading behavior

248th National Meeting (2014)

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