An experimental study of surfactant effects on spilling breakers

Abstract

The dynamics of spilling breakers in the presence of surfactants were studied experimentally. The spilling breakers were produced from Froude-scaled mechanically generated dispersively focused wave packets with average frequencies of 1.15, 1.26 and 1.42 Hz. Separate experiments were performed with the same wave-maker motions in clean water and in water with various bulk concentrations of the soluble surfactants sodium dodecyl sulfate (SDS) and Triton X-100 (TX). For nearly all surfactant conditions, the surface-pressure isotherm, equilibrium surface elasticity and surface viscosity were measured in situ in order to characterize the dynamic properties of the free surface. In clean water, all the waves considered herein break without overturning of the free surface. This breaking process begins with the formation of a bulge on the forward face of the wave crest and capillary waves upstream of the leading edge of the bulge (called the toe). After a short time, the flow separates under the toe and a turbulent flow is developed while the toe moves rapidly down the wave face. During the toe motion, a train of ripples appears between the toe and the crest and this train of ripples is swept downstream. In the presence of surfactants, the bulge shape is modified and its size generally decreases with increasing surfactant concentration. The capillary waves found upstream of the toe in the clean-water case are dramatically reduced at even the lowest concentrations of surfactants. With surfactants, the start of the breaking process is still initiated when the toe begins to move down the forward face of the wave. The pattern of ripples generated between the toe and the crest of the wave during this phase of the breaking process varies with the concentration of surfactant. It was found that the temporal history of the vertical distance between the toe and the wave crest scales with the nominal length $(\sigma_0/\rho g)^{1/2}$ while the bulge length from toe to crest scales with the nominal length $(\mu_s/\rho \sqrt{g})^{2/5}$, where $\sigma_0$ and $\mu_s$ are the ambient surface tension and the surface viscosity, respectively.

Capillary effects on wave breaking

“…The overturning entrainment is avoided either because the slope of the wave is too low, resulting in a gentle spilling wave (Rapp & Melville 1990), or due to stabilization by surface tension effects (Duncan et al 1999;Duncan 2001). These waves have been observed and described in laboratory experiments (Rapp & Melville 1990;Lin & Rockwell 1995;Duncan et al 1999;Qiao & Duncan 2001;Liu & Duncan 2003, 2006 and numerically by Iafrati & Campana (2005). This regime is also obtained in our simulations as illustrated in figure 4, for a relative high initial steepness and low Bond number (Bo = 25, = 0.45).…”

“…Thus it appears that for this intermediate steepness, a small change in the surface tension can cause qualitative changes in the wave dynamics. These types of wave structure changes have been explored experimentally by adding surfacants to water (Liu & Duncan 2003, 2006. Air entrainment by breaking waves has been investigated experimentally (Lamarre & Melville 1991, 1994Loewen et al 1996;Blenkinsopp & Chaplin 2007;Rojas & Loewen 2010) but carefull comparisons would require the extension of the present simulation to three-dimensions.…”

“…The understanding of dissipation induced by breaking waves at various scales has been the topic of intensive research in recent decades, combining field work in the open ocean (Melville 1996;Melville & Matusov 2002;Veron et al 2008;Sutherland & Melville 2013), laboratory experiments on wave breaking (Melville & Rapp 1985;Tulin & Waseda 1999;Rapp & Melville 1990;Banner & Peirson 2007;Drazen et al 2008), and the dissipation induced by smaller scales involving surface tension, like micro-breaking, spilling breaking (Duncan et al 1999;Duncan 2001;Liu & Duncan 2003, 2006 and parasitic capillary waves (Perlin et al 1993;Jiang et al 1999;Caulliez et al 1998;Su 1982;Caulliez 2013). The importance of small scales, O(1 − 10) cm in the global ocean wave energy fluxes is still an open question and needs further investigation.…”

“…The influence of capillary effects on the wave breaking dynamics is discussed extensively experimentally (e.g. the review of Duncan (2001)) while the significance of surface tension effects can be experimentally studied by adding surfactants that are able to modify the breaking kinematic and dissipation properties (Liu & Duncan 2003, 2006.…”

On the Mach reflection of a solitary wave: revisited

“…To identify the air-water interface, we traced image pixels in the vertical direction from top to bottom; the interface was determined where the gradient of the light intensity reaches a maximum. A similar procedure was used by Liu & Duncan (2006) for their laboratory experiments on wave-breaking processes. One of the difficulties associated with the LIF method for measuring long waves is the limitation in resolution.…”