Insoluble surfactant spreading along thin liquid films confined by chemical surface patterns

Abstract

We conducted a combined experimental and numerical study of the spreading of insoluble surfactants on spatially confined thin liquid films. We found that the spreading dynamics can locally be represented by a power-law relation x∼t(α). We determine the time evolution of the liquid film thickness and the corresponding spreading exponents α both from experiments using interference microscopy and numerical finite element simulations. The lateral confinement induces non-uniform height- and surface velocity profiles, which manifest themselves in a pronounced transition of the evolving rivulet morphology. Excellent agreement between experimental and simulation results has been achieved.

Soluble surfactant spreading on spatially confined thin liquid films

“…3,7,8,[15][16][17]19,20,25,26,33,34,36,45,46,48,51,53 It was observed that film thinning occurred in the vicinity of the deposited surfactant as well as film thickening and the formation of a rim near the surfactant leading edge. The rim position x rim (t) follows a power law behavior x rim z t a , where a is the so-called spreading exponent.…”

“…Several local maxima in h max ( t) are observed. As in the case of insoluble surfactant, 45 the maximum around t ¼ 20 is due to the decay of transverse surface tension gradients. Fig.…”

“…Lateral surface tension gradients occur during surfactant spreading on uncontaminated rivulets as a consequence of the non-uniform transverse height profile. 45 Liquid is pushed from the center of the rivulet to its edges, corresponding to a flattening of the height profile. When an insoluble contaminant is present, the contaminant concentration in the vicinity of the rim is higher near the rivulet edges than in the rivulet center, as illustrated in Fig.…”

“…A variety of spreading exponents were reported for different material systems and geometrical configurations: a ¼ 0.5 for planar spreading, 3,7 exponents around a ¼ 0.25 for axisymmetric spreading with large Peclet numbers 19,20 and values around 0.33 for spreading along chemically defined liquid rivulets. 45 In technological applications, surfactants are often soluble in at least one phase. The spreading dynamics of soluble surfactants at air-liquid and liquid-liquid interfaces has been studied in ref.…”

Self-propelling surfactant droplets in chemically-confined microfluidics – cargo transport, drop-splitting and trajectory control

“…17 Related propulsion mechanisms for drop transport along liquid-air 18 as well as liquidsolid interfaces 19 have been studied in recent years. Our chemical patterning procedure, † based on photolithography and self-assembling mono-layers of 1H,1H,2H,2H-perfluorooctyltrichlorosilane was previously described, 20 along with the subsequent cleaning step. Using this procedure we created chemically patterned surfaces on Si-substrates and borosilicate glass slides (thickness 150 mm), with hydrophilic stripes of length L $ 220 mm and width w ¼ 1.5 mm on a hydrophobic background, as sketched in Fig.…”