2.2.2. Dynamic Surface Tension

For very short and intermediate adsorption times (from milliseconds to hundreds of seconds), dynamic surface tension, γ(*t*), was measured by means of a homemade apparatus using the maximum bubble pressure technique (MBP). The setup is similar to the one described elsewhere [10,15] and is shown in Figure 1. The technique is based on the measurement of the maximum pressure needed to form a bubble at the tip of a capillary immersed in the surfactant solution. The pressure is related to the surface tension by the Young-Laplace equation [16], ΔP = 2γ/r being r the radius of the capillary. The

device consists of a peristaltic pump which pressurizes a 1 dm3 air reservoir, an electrically controlled needle valve (Aalborg PSV1S-VA with its driver module PSV-D, Organgeburg, NY, USA) that controls the air flow to a micro-pipette tip (Sartorius Optifit 791000, Buenos Aires, Argentina) and acts as the capillary, and is placed 5 mm below the solution's surface. The pressure difference, ΔP, inside the tip was measured with a differential pressure transducer (Cole-Palmer GY-98073-08, Vernon Hills, IL, USA) and transmitted using USB to a PC by a Teensy 3.2 microcontroller board. The Teensy board also senses and controls the reservoir pressure (pressure transducer Ashcroft G2-7-M02-15-G2-30G, Stanford, CT, USA) and the needle valve opening. The tip makes an angle of 30◦ with respect to the interface and is discarded after each measurement. Each tip is calibrated with pure water prior to measurement by measuring the ΔP as a function of depth, which is precisely controlled by means of the step-motor.

**Figure 1.** (**a**) Scheme of the Maximum Pressure Apparatus. (**b**) Picture of the device showing the tip and the step-motor.
