The effect of surfactants on droplet generation in a plunging breaker

An experimental study is conducted to compare droplet generation in a deep-water plunging breaker in filtered tap water (referred to as case TAP, ambient surface tension σ₀ = 72 mN/m) and in the presence of two bulk concentrations (C_TX) of the soluble surfactant Triton X-100: TX1, where C_TX = 2.1μmol/L and σ₀ = 69 mN/m; and TX6, where C_TX = 193 μmol/L and σ₀ = 35 mN/m. In the TX6 case, C_TX is close to the critical micelle concentration (CMC). The breakers are generated by a programmable wave maker that is set with a single motion profile that produces a highly repeatable dispersively focused quasi-2D wave packet with a central frequency of f₀ = 1.15 Hz and a corresponding wavelength of λ₀ = 1.18 m (by linear theory). The droplets are measured with an in-line cinematic holographic system operating with measurement volumes that span the width of the tank. The positions, diameters (d ≥ 100 μm), times and velocities of droplets generated in these three cases are measured as the droplets move up across a prescribed horizontal measurement plane. It is found that the surfactants have strong effects on the droplet production mechanisms and the droplet number, diameter, and velocity distributions. One example is the droplets produced by the closure of the crater generated between the upper surface of the plunging jet and the splash that it produces. This crater closes rapidly in the TAP and TX6 cases but not in the TX1 case, resulting in much fewer droplets produced in TX1 than in TAP and TX6.