# Storage Stability of Bimodal Emulsions vs. Monomodal Emulsions
## 1 Introduction  
Emulsions are dispersed systems consisting of two immiscible liquids; one of them is the continuous phase, which contains the other liquid or internal phase, dispersed as small droplets. These systems are thermodynamically unstable and have a certain kinetic stability conferred by the presence of a surfactant absorbed on the oil/water interface. Surfactants are chemical compounds with a surface activity which, when dissolved in a liquid, especially water, lowers its interfacial tension between the immiscible liquids, by preferential adsorption at the vapor/liquid surface or other interfaces, and facilitate dispersion of one liquid into the other [1; 2]. Particle size and particle size distribution are important variables to consider and are controllable with formulation, raw materials, and the equipment used to manufacture the emulsion [3]. Many of the processes of breaking and curing are directly dependent on particle size and particle size distribution [4]. The importance of particle size in emulsions has been discussed in many papers [3; 4; 5]. It is a determinant of emulsion stability, coating, break rate, and cure rate. Methods to improve particle sizing of emulsions by formulation and adjustment of asphalt chemistry are described in the literature [5; 6]. These methods usually involve improvement of the dispersing phase, doping of asphalt with surfactants, and tailoring asphalt composition and optimization of manufacturing conditions. The intention is to improve formation and dispersion of asphalt particles by the mill and stabilize the resulting emulsion [2]. The mean droplet size and the particle size distribution obtained can be controlled by changing the parameters of the formulation: rotational speed, mixing time, and surfactant type and concentration. The span of the drop size and the Sauter mean diameter become larger when the dispersed phase volume fraction becomes higher and the rotor speed becomes lower [3]. The type of surfactant used in the manufacture of the emulsion has a big effect on the speed of the breaking of an emulsion, so storage stability is one of the most important parameters to take into account in the formulation of bituminous emulsions. Bitumen in water emulsions are complex fluids. Their stability is governed by intermolecular forces, a result of a balance of repulsive and attractive forces. The formulator skillfully must understand and balance these forces such that the emulsion can be produced consistently, stored, pumped, transported, and applied by the practitioner in the field without experiencing any downtime in the operation. This difficult behavior could be reached, among other things, with a correct command of the formulation. If we want that the emulsion to have the same properties at the moment of application from the moment of manufacture, they have to be stable emulsions, or have good storage stability. The stability of the emulsion determines its appropriate use. Industrial manufacture of asphalt emulsion uses custom made equipment to provide thorough mixing capability to provide the fineness and stability of dispersion required to meet the desired asphalt emulsion properties. Once the asphalt emulsion has been manufactured for an application, tests must be conducted on the material to ensure it has the appropriate stability for storage and shipping. The weight of asphalt is very close to that of water, which helps in the ability of the asphalt to not settle to the bottom or cream to the top of the final product. Tests such as settlement and storage stability are done at 24 h and five days after manufacture to ensure the product maintains a good consistency [2]. The droplets of an emulsion are subjected to Brownian motion, which is the random motion observed in particles in a fluid (liquid or gas) as a result of collisions with the molecules of said fluid [4; 5]. Brownian motion is an inherent movement in emulsions. The dispersed particles move inside the fluid.