# Storage Stability of Bimodal Emulsions vs. Monomodal Emulsions
## 1 Introduction
We must avoid, as far as possible, that this movement be abrupt or excessively fast, as an emulsion particle could be exchanged with another, fusing and resulting in larger droplet sizes. Industrial process, it is usual to pass the emulsion through a cooler in order to decrease Brownian motions. The stability of the emulsion will depend on how they interact with each other when encountered [2]. When the globules approach, due to the increase of concentration, by sedimentation if the increase of concentration is at the bottom, or creaming if the increase of concentration is at the top, the possibility of contact is much higher. Because of packaging or of friction between beads, some emulsifier molecules are removed from the surface and these unprotected areas are sensitive points to cause the emulsion to rupture [6; 7]. During the storage of an emulsion, creaming or sedimentation takes place in a first step. Subsequently, flocculation occurs; it is characterized by the contact between globules, but they still are partially protected by the film of the emulsifier and, therefore, they maintain their shape. Then coalescence appears in what is already an irreversible process. After the first coalescence, these phenomena are accelerated resulting in the fracture and separation of the phases of the emulsion (Figure 1) [8; 9]. The main factors that influence the storage stability of an emulsion are viscosity and average droplet size [2]. These two parameters are evaluated in this paper. Bituminous emulsions form the basis for many paving applications in our asphalt industry, including driveway sealants, cold-pour crack sealants, and roofing emulsions or the recent development of half warm mix asphalt. Their rheological properties, such us viscosity, often dictate the uses for which they are suitable. For example, the viscoelastic properties of a slow setting versus a rapid-setting emulsion are different. We expect one day to be able to use rheological properties of bituminous emulsions to predict their success or failure in their respective applications. Even when rheological properties are not critical in the final product, they influence the workability of the emulsion as it is applied in the field [2]. Emulsion viscosity is a main test for the application of products. In a mixing application of dense graded aggregates, a thinner viscosity of emulsion is usually required. For an opengradedapplication, a thicker emulsion is usually desired to get the appropriate film thickness. An asphalt emulsion used for bonding layers of asphalt needs to be thin enough to make a uniform layer and may need to be diluted with water to achieve the desired properties. For seal coating with aggregate chips, the emulsion needs to be thick enough to form the appropriate layer of emulsion for the size of chip while not forming drill marks or running off the road. In order to measure the stability of the emulsions [PERSON] law should be considered. [PERSON] law evaluates sedimentation time as a function of mean particle size of the emulsion. In general, [PERSON] law is valid in the movement of small spherical particles moving at low speeds but only for monomodal emulsions, as we will demonstrate in this paper [2; 10; 11; 12]. If the particles are falling vertically into a viscous fluid due to their own weight, their falling rate or settling can be calculated by equating the frictional force with the apparent weight of the particle in the fluid. The speed of this phenomenon, still reversible, is governed by [PERSON] law [13; 14; 15; 16]:  
\[\mathrm{V}=\frac{2\mathrm{a}^{2}(\rho-\rho_{0})\mathrm{g}}{9\eta}\mathrm{V}= \frac{2\mathrm{a}^{2}(\rho-\rho_{0})\mathrm{g}}{9\eta} \tag{1}\]  
where V is the particles falling rate, a is the globules ratio, \(\rho\) is the bitumen density, \(\rho_{0}\) is the water density, \(\eta\) is the emulsion viscosity, and g is acceleration.