Influence of shear forces and surfactant deployment on foam production

To achieve stable foams it was crucial to obtain small bubble diameters as described by Cooke and Hirt.^[238] The main processing parameters were adequately adjusted at room temperature, such as the aeration in form of the utilized air flow rate and air pressure, as well as, the shear stress in the applied form of the propeller’s rotations per minute.

Smaller bubbles with larger specific surface-area-to-volume ratios showed reduced tendencies to collapse.^[239]

As described by Wenzel et al., foam bubble diameters decreased under high shear in motion.^[240] Similar results were published by Parikh.^[241] Since a propelling unit was used in this work and no tubular set-up, increased shearing was applied by rotational forces. It could be confirmed that maximized shear rates and therefore revolutions per minute in-crease the blow ratio. It is likely that this effect continues for even higher shear rates, but in the presented setup 1000 rpm was the highest applicable rotational speed. In terms of the aeration, both intermediate flow rate and pressure were beneficial. This effect seems antithetic to common sense, since foam generation in simple devices, such as syphons, depends mainly on high pressure. The set-up is not closed, and therefore, the pressure is not maintained inside the liquid volume. Created air bubbles pass through the medium and accumulate at the surface. Trapping these new bubbles in the foamed medium high-ly depends on the shear rate as shown by Drenckhan and Saint Jalmes.^[242] Politova et al. described the bubble size in a foam created in a planetary mixer.^[243]

The air entrapment in the foam is limited, and therefore its volume-growth is limited. Al-so, the air entrapment and air volume ratio is relative to the gained bubble sizes.

Transferred to the presented setup, these findings clearly affirmed the results. The em-ployed propeller construction and maximal applied rotational speed result in a limited shear force. When more air is taken into the system and the velocity bubble creation is larger than that of bubbles bursting on the surface (Figure 4.1 A), the bubbles accumu-late inside the foam or at the surface. As soon as the inner pressure of these bubbles exceeds the membrane surface tension, these bubbles burst. The emerging pressure force may even destroy fractions of the desired foam as a result (Figure 4.1 B). Reducing this effect by using special foaming agents and detergents in combination with setting up optimal working parameters is of utmost importance to yield a homogeneous silk protein coating distribution in stable foam with high integrity as well as a small bubble structure (Figure 4.1 C).

Figure 4.1: Schematic illustration of laboratory scale foaming process with an air flow rate, yielding no foam (A), exceeding air flow, air bubble aggregation and collapsing foam (B) and ideal air flow rate and accumulating foam (C).

The surfactant fulfills two main purposes. First, it enables the foamability itself by reduc-ing the aqueous surface tension. On the surface of water-based liquids, water molecules

A B C

and the molecule cluster (Figure 4.3 A) arrange in a way that attractive forces are orient-ed to the inside. Therefore, the molecules on the surface are excessively contractorient-ed.

Surfactant molecules may accumulate on the liquid’s surface and envelop the water molecules, reducing this tension significantly. Subsequently, air bubbles also might be entrapped in such agglomerated water-surfactant conjunctions, increasing essentially the systems foamability.

The second important effect of the surfactant is the stabilization of the silk protein mole-cules. Aqueous solutions of spider silk proteins are produced via dialysis. Dissolved in solution eADF4(C16) molecules are unfolded. Disturbing factors such as shear forces or the thermal energy by stirring friction enforce misfolding and aggregation. As reported by Gleuwitz, the surfactant also stabilizes the silk protein molecules in solution and prevents agglomeration.^[236] The produced surfactant concentration herein displays a minimum value attuned to the utilized protein concentration. Below this value, precipitation of silk protein molecules is reported. Even though, a higher surfactant concentration is not aimed for, since a well-distributed silk protein concentration in the created foam is de-sired and an exceeding amount of surfactant molecules would block the silk. An exceed-ing employment of surfactant also increases the likelihood of producexceed-ing silk protein free bubbles (Figure 4.3 B) and a subsequent inhomogeneous foam coating. The employed surfactant is a branched non-ionic iso-tridecyl-alcohol, displayed in Figure 4.2.^[244] The hydrophilic negatively charged hydroxyl group likely interacts with the positively charged amino terminal group of eADF4(C16) silk protein, as well as, with the polar H-Groups of the water molecules. The hydrophobic carbohydrate chains of the surfactant molecule might either interact with the hydrophobic parts of the silk molecule chains, which are not protected, or stick to the outside, yielding a micelle-like structure.

Figure 4.2: Ultravon Jun surfactant main ingredient, Iso-tridecyl-alcohol.

The ideal foam bubble (Figure 4.3 C) carries a membrane of surfactant enveloped silk protein - water conjugations on its surface. Ideally a wet silk protein film with surfactant is

deposited on the textile fiber surface when the bubble collapses. Consequently, by the applied vacuum the flexible wet film is still able to penetrate deeper into the yarn struc-ture. After drying, the film hardens and acts as protective layer.

A

Figure 4.3: Schematic illustration of a water (6) molecule cluster (A), a foam bubble with wa-ter cluswa-ter-surfactant (iso-tridecyl-alcohol) conjugations (B), and an ideal bubble (C) carrying a membrane of surfactant enveloped silk protein - water conjugations on its surface.

4.1.2 Adhesion behavior of spider silk proteins on different yarn fiber