Final Report Summary - DEFFED (Design of froths and foams with entailed durability)
Project objectives
This project aims at creating methodology for producing of foams with entailed foaminess and durability. Generally, the very generation of foam can be split to two basic stages:
(i) formation of bubbly flow in the surfactant solution;
(ii) pre-concentration of the bubbles from the bubbly flow onto the air/water interface thus causing the formation of rising foam cap.
The basic difference between the known methods for generation of foam consists mainly in the first step. For example, the sparging of gas into the surfactant solution is known as the most productive way to generate foam, because no mechanical work is required for entrapping of ambient air in form of bubbles. In this sense, our first efforts (Karakashev, 2010; Karakashev et al., 2011a) were focused on studying the dynamic contact between bubbles within the bubbly flows. It was clearly shown that the critical Weber number, Wecr, is responsible for the coalescence of striking bubbles. Even a very small amount of surfactant added, significantly reduces the value of Wecr. Thus was shown that traces of surfactants prevent the coalescence of the bubbles from the bubbly flows. However under such conditions, the bubbles usually coalesce at the air/water interface due to their enormous pre-concentration. The main question here was why the bubbles do not coalesce in the bubbly flow but only in the region of accumulation close to air/water interface? In this zone, the bubbles are pressed toward each other and move in different directions in the same time. Hence they are subjected to stresses pressing them to coalesce. The ability of the bubbles to oppose these stresses is expressed in their elastic moduli. For this reason we had to investigate how the elastic moduli of the foam bubbles affect the stability of foams (Karakashev et al., 2011b; Karakashev et al., 2010a; Karakashev et al., 2010b). A direct correlation was shown between the values of the elastic moduli and foam durability. Once the foam is generated, it starts draining and decaying. We had to study these two processes in more detail. For this reason, the foam pressure drop method was applied to study them (Kruglyakov et al., 2010a; Kruglyakov et al., 2010b). It was established that foams stabilised by ionic surfactants drain slower than such ones stabilised by non-ionic surfactants. The larger the foam bubbles, the stronger the influence of their coalescence on the foam drainage. The next step from the execution of the project was to add particles into the surfactant solution and to test the behaviour of foam films and foams. We set two problems, which were poorly studied in the literature the effects of particles shape and agglomeration on foam stability. It was found (Karakashev et al., 2011c) that needle-like particles stabilise the foams significantly better than spherical particles. In addition, the network of micro silica particles stabilises foams and foam films significantly better compared to this one of the nano-particles (Vilkova et al., 2011; Vilkova et al., 2012). The next task, which we set, was to find correlations between the molecular structure of the surfactants, their concentrations and the foaminess and foam stability. Meanwhile, it was established that air humidity significantly affects the foaminess and the foam durability (Li et al., 2012). The larger the humidity, the longer the lifetime of the foam is, and vice versa. In this sense, two basic types of foams exist transient (fast decaying) and tenacious (long standing). Both of them have large industrial applications under dynamic conditions. For this reason, we investigated both them as pneumatic foams.
The experimental data (Karakashev et al., 2012a; Karakashev et al., 2012b) on growth of tenacious and transient showed that:
1. The rates of foam growth and gas delivery coincide until a certain critical value (Qcrit) of the gas flux, at which the foam growth exceeds the gas delivery rate;
2. The smaller the value of the dynamic surface tension, the larger the difference between the rates of the foam growth and the gas delivery are (at Q?Qcrit);
3. The increase of the gas flux to values substantially larger than the critical one causes significant exceedance of the foam growth rate as compared to the gas delivery rate, but the dependence of this exceedance on the dynamic surface tension weakens;
4. There is a second critical value of the gas delivery rate at which the exceedance of foam growth rate compared to the gas delivery rate does not depend on the surface tension anymore;
5. The experimental data on transient foams stabilised by aqueous solutions of three homologue surfactants but with identical equilibrium surface tension showed that the foaminess of the stronger surfactant is larger;
6. A new parameter expressed in ratio between the foaminess and rate of foam decay foam production was defined;
7. A correlation between the surfactant molecular structure, surfactant concentration, gas delivery rate and foam production was shown as a basis for modelling and design of foam with entailed foaminess and foam durability.
This project aims at creating methodology for producing of foams with entailed foaminess and durability. Generally, the very generation of foam can be split to two basic stages:
(i) formation of bubbly flow in the surfactant solution;
(ii) pre-concentration of the bubbles from the bubbly flow onto the air/water interface thus causing the formation of rising foam cap.
The basic difference between the known methods for generation of foam consists mainly in the first step. For example, the sparging of gas into the surfactant solution is known as the most productive way to generate foam, because no mechanical work is required for entrapping of ambient air in form of bubbles. In this sense, our first efforts (Karakashev, 2010; Karakashev et al., 2011a) were focused on studying the dynamic contact between bubbles within the bubbly flows. It was clearly shown that the critical Weber number, Wecr, is responsible for the coalescence of striking bubbles. Even a very small amount of surfactant added, significantly reduces the value of Wecr. Thus was shown that traces of surfactants prevent the coalescence of the bubbles from the bubbly flows. However under such conditions, the bubbles usually coalesce at the air/water interface due to their enormous pre-concentration. The main question here was why the bubbles do not coalesce in the bubbly flow but only in the region of accumulation close to air/water interface? In this zone, the bubbles are pressed toward each other and move in different directions in the same time. Hence they are subjected to stresses pressing them to coalesce. The ability of the bubbles to oppose these stresses is expressed in their elastic moduli. For this reason we had to investigate how the elastic moduli of the foam bubbles affect the stability of foams (Karakashev et al., 2011b; Karakashev et al., 2010a; Karakashev et al., 2010b). A direct correlation was shown between the values of the elastic moduli and foam durability. Once the foam is generated, it starts draining and decaying. We had to study these two processes in more detail. For this reason, the foam pressure drop method was applied to study them (Kruglyakov et al., 2010a; Kruglyakov et al., 2010b). It was established that foams stabilised by ionic surfactants drain slower than such ones stabilised by non-ionic surfactants. The larger the foam bubbles, the stronger the influence of their coalescence on the foam drainage. The next step from the execution of the project was to add particles into the surfactant solution and to test the behaviour of foam films and foams. We set two problems, which were poorly studied in the literature the effects of particles shape and agglomeration on foam stability. It was found (Karakashev et al., 2011c) that needle-like particles stabilise the foams significantly better than spherical particles. In addition, the network of micro silica particles stabilises foams and foam films significantly better compared to this one of the nano-particles (Vilkova et al., 2011; Vilkova et al., 2012). The next task, which we set, was to find correlations between the molecular structure of the surfactants, their concentrations and the foaminess and foam stability. Meanwhile, it was established that air humidity significantly affects the foaminess and the foam durability (Li et al., 2012). The larger the humidity, the longer the lifetime of the foam is, and vice versa. In this sense, two basic types of foams exist transient (fast decaying) and tenacious (long standing). Both of them have large industrial applications under dynamic conditions. For this reason, we investigated both them as pneumatic foams.
The experimental data (Karakashev et al., 2012a; Karakashev et al., 2012b) on growth of tenacious and transient showed that:
1. The rates of foam growth and gas delivery coincide until a certain critical value (Qcrit) of the gas flux, at which the foam growth exceeds the gas delivery rate;
2. The smaller the value of the dynamic surface tension, the larger the difference between the rates of the foam growth and the gas delivery are (at Q?Qcrit);
3. The increase of the gas flux to values substantially larger than the critical one causes significant exceedance of the foam growth rate as compared to the gas delivery rate, but the dependence of this exceedance on the dynamic surface tension weakens;
4. There is a second critical value of the gas delivery rate at which the exceedance of foam growth rate compared to the gas delivery rate does not depend on the surface tension anymore;
5. The experimental data on transient foams stabilised by aqueous solutions of three homologue surfactants but with identical equilibrium surface tension showed that the foaminess of the stronger surfactant is larger;
6. A new parameter expressed in ratio between the foaminess and rate of foam decay foam production was defined;
7. A correlation between the surfactant molecular structure, surfactant concentration, gas delivery rate and foam production was shown as a basis for modelling and design of foam with entailed foaminess and foam durability.