**4.1. Contact angle and surface tension**

The attachment of particles at gas liquid interfaces occurs when particles are not completely wetted or, in other words, are partially hydrophobic. This enables the production of highvolume stable foam, which produces porous ceramics after drying and sintering. Partially hydrophobic particles remain predominantly in the liquid phase and exhibit a contact angle <90°. Therefore, controlling contact angles of the particles at the interface is important since the angles modify the wettability of the particles by changing their hydrophobicity, as shown in (Figs. 8-10). Generally, lower contact angles improve the wettability. Different contact angles can be achieved by imparting different hydrophobic molecules commonly known as surfac‐ tants.

It is shown from Fig. 8 that the average contact angle of the d50 ~ 40 nm Al2O3 suspension decreased from 84° to 67° with the increased SiO2 content (1.0 mole ratios in the Al2O3 suspension). Also, the increasing SiO2 content produced lower adsorption free energy due to the higher interparticle attraction, increasing the viscosity. The suspensions with mol ratios of SiO2 between 0.25 and 0.5 in the suspension show higher levels of attachment energy, resulting in highly stable foam in the sintered porous ceramics. Also, contact angle of around 70°-75° for the nanoparticle suspension leads to better wet foam stability and can give surface tensions of 21-33 mNm-1. The required partial hydrophobization of the particles occurs at this point, which leads to porous ceramics with higher porosity.

Fig. 9 shows the effect of suspension added for the mullite phase on the contact angle and surface tension of the aqueous suspensions. From this graph, we can see the suspension exhibits contact angles of 46°-55°, which enables high wet foam stability, as that indicates partial hydrophobization of particles has taken place. We observed that for all the evaluated samples, the surface tension of suspensions decreases, upon increasing the vol% of suspension added for the mullite phase. This can be explained by an increase in surface hydrophobicity of the particles with increasing particle concentration.

The difference in the Laplace pressure between bubbles of distinct sizes (R) leads to bubble disproportionation and Ostwald ripening because of the steady diffusion of gas molecules from smaller to larger bubbles over time. This process can be slowed by using surfactants or particles adsorbed at the interface, which decrease the interfacial energy. Wet foam's stability is also related to the degree of hydrophobicity achieved from the surfactant, which replaces part of the highly energetic interface area and lowers the free energy of the system, leading to an apparent reduction in the surface tension of the suspension [49]. Stability also depends on surface charge screening, the electrical diffuse layer around a particle's surface not sufficiently thick to overcome the attractive van der Waals forces between particles. Overcoming the van der Waals attractions requires a stable hydrophobizing mechanism (examined above).

Therefore, experiments were conducted as per reported theoretical explanations [18].

interfacial areas.

66 Advanced Ceramic Processing

tants.

**4. Results and discussions**

**4.1. Contact angle and surface tension**

which leads to porous ceramics with higher porosity.

These actions' combined effects may collapse the foam within minutes after air incorporation. Foams' life times have been increased from several hours to days and months by the adsorption of the short chain amphiphilic molecules [50], while only a few minutes or hours' stabilization results from the use of long-chain surfactants or proteins at the air-water interface [35]. Unlike other particle-stabilized foams [2], these foams percolate throughout the whole liquid phase and exhibit no drainage over days and months [49] due to the high concentration of modified particles in the initial suspension, which allows for the stabilization of very large total air-water

The attachment of particles at gas liquid interfaces occurs when particles are not completely wetted or, in other words, are partially hydrophobic. This enables the production of highvolume stable foam, which produces porous ceramics after drying and sintering. Partially hydrophobic particles remain predominantly in the liquid phase and exhibit a contact angle <90°. Therefore, controlling contact angles of the particles at the interface is important since the angles modify the wettability of the particles by changing their hydrophobicity, as shown in (Figs. 8-10). Generally, lower contact angles improve the wettability. Different contact angles can be achieved by imparting different hydrophobic molecules commonly known as surfac‐

It is shown from Fig. 8 that the average contact angle of the d50 ~ 40 nm Al2O3 suspension decreased from 84° to 67° with the increased SiO2 content (1.0 mole ratios in the Al2O3 suspension). Also, the increasing SiO2 content produced lower adsorption free energy due to the higher interparticle attraction, increasing the viscosity. The suspensions with mol ratios of SiO2 between 0.25 and 0.5 in the suspension show higher levels of attachment energy, resulting in highly stable foam in the sintered porous ceramics. Also, contact angle of around 70°-75° for the nanoparticle suspension leads to better wet foam stability and can give surface tensions of 21-33 mNm-1. The required partial hydrophobization of the particles occurs at this point,

**Figure 8.** Contact angle and surface tension of colloidal suspension with respect to different mole ratio of SiO2.

In Fig. 10, the hydrophobization achieved via amphiphile adsorption was confirmed by contact angle measurements of the aqueous suspensions. As we can see, the 0.05-mol/L concentration of propionic acid was not sufficient enough to impart particle hydrophobicity, which results in unstable foams. From this graph, we can conclude that a contact angle of 65°-72° produces required particle hydrophobicity which enables high wet foam stability.

In Fig. 11, the degree of particle hydrophobization achieved by imparting different concen‐ tration of amphiphile was investigated with the help of surface tension measurements. The surface tension of suspensions containing 30 vol% particles and different concentration of amphiphiles is shown in Fig. 11. A decrease in surface tension upon increasing the amphiphile concentrations is observed for all the evaluated suspensions. The reduction in surface tension results from the adsorption of free amphiphile molecules to the air-water interface. The middle and short chain amphiphiles, i.e., butyric acid and valeric acid, respectively, impart relatively low surface energy, which enables sufficient hydrophobicity on the particle surface than the shortest chain amphiphile, i.e., propionic acid, does.

**Figure 9.** Contact angle and surface tension of Al2O3-TiO2equimolar suspension, with respect to different vol% of 3:2 mole ratio of Al2O3-SiO2 suspension added for the mullite phase32

**Figure 10.** Contact angle of suspension with respect to different concentrations of amphiphiles.4

**Figure 11.** Surface tension of suspension with respect to different concentration of amphiphiles.4
