Retarding spreading of surfactant drops on solid surfaces: Interplay between the Marangoni effect and capillary flows

The characteristics of an aqueous drop spreading on a solid surface are the keys to many deposition processes, including coating, printing, enhanced oil production, and surfactant replacement therapies. It is generally believed that the addition of surface-active materials increases the rate of the drop spreading. We report on an unexpected phenomenon whereby the initial spread of surfactant drops is impeded by the Marangoni stresses, resulting in a large increase in the total spreading time. This is demonstrated by an experimental study of the early-time regimes of surfactant-laden drops spreading on a hydrophilic solid surface, submerged in a second immiscible viscous liquid. Remarkably, we find that the surfactants delay the initial fast motion of the three-phase contact lines. The nonuniform distribution of the surfactants at the interface generates the Marangoni stresses before the drop-solid contact that suppresses the film drainage and the expansion of the droplet. Surfactant solutions with 0.1 and 1 critical micelle concentrations wet the surface, with their radius growing according to $r\sim t^{0.5}$ for very short timescales, as opposed to the viscous regime for which $r\sim t$. Our research can provide a guideline for the design of systems with controllable and desirable wetting dynamics.

Figure 1

(a) Schematic of the experimental setup. The bottom-view images were used for measuring the radius $r\left(t\right)$ of the spreading area, and side-view images were used for detecting the drop shape. (b) The IFT and contact angle values of the samples. Triangles, spheres, squares, and inverted triangles show the data for water, SDS 0.1 CMC, SDS 1 CMC, and the water-ethanol mixture, respectively. Solid and empty symbols represent the IFT and the contact angle data. (c) Dynamic IFT of the 1 CMC solution of SDS.

Figure 2

(a) Evolution of radius $r\left(t\right)$ of the drop spreading at four concentrations of the surfactant, 0, 0.01, 0.1, and 1 CMC, and a mixture of water-ethanol in, respectively, black, blue, green, red, and gray on the log-log plot. (b) Evolution of the measured apparent exponent $\alpha$. The zone between the solid lines represents the initial spreading regimes of water, SDS 0.01 CMC, and water-ethanol that have spreading exponents close to 1. The zone between the dashed lines shows the initial regimes of SDS 0.1 and 1 CMC that have spreading exponents close to 0.5. (c) The velocity of the three-phase contact line as a function of spreading radius $r$. Compared to water, the addition of the surfactant at 0.1 and 1 CMC decreases the initial spreading velocity by 1 order of magnitude. (d) Dimensionless velocity as a function of the dimensionless radius.

Figure 3

Images of the drops of (a) water and (b) surfactant (with 1 CMC solution of SDS) for three stages of the process, namely, descending, resting, and the onset of spreading at the film rupture. The selected areas in the images show the flattened aqueous phase-oil film interface at the bottom of the SDS drop.

Figure 4

(a) and (b) Effect of surfactant before the drop-solid contact. Its addition generates surface tension gradients and, hence, the Marangoni flow hindering the film drainage. Distribution of the surfactant molecules (red head and dotted black tail) at the drop interface is schematic. The surfactant molecules inside the SDS drop are not shown. Green arrows indicate the interfacial and internal flows in the drop phase, induced by the surface tension gradient. Black arrows depict the flow distribution in the oil film beneath the drops.

Figure 5

Dynamic IFT of the SDS 0.1 CMC droplet and the water-ethanol droplet in the surrounding oil phase. The IFT of the water-ethanol droplet remains constant over time. However, the IFT of the SDS 0.1 CMC droplet is a function of time. Both systems have the same equilibrium IFT.

Figure 6

(a) Effect of initial drop volume on the spreading regimes. A 1 CMC solution of SDS drop spreading (scaled with the initial drop radius) with different volumes from 2 to $10\mu \mathrm{l}$. (b) Effect of the surrounding liquid viscosity on spreading regimes.