# Storage Stability of Bimodal Emulsions vs. Monomodal Emulsions
## 1 Introduction
According to the above equation, [PERSON] law refers to the frictional force experienced by spherical objects moving in the interior of a viscous fluid in a lamellar regime with low Reynolds numbers, from which we can deduce that the sedimentation velocity is determined by two important factors: the mean droplet size and the viscosity of the emulsion. So, focusing on [PERSON] law, the lower the average droplet size of the emulsion, the lower the sedimentation rate; and the more viscosity the emulsion has, the lower the sedimentation rate [17]. This rule is fulfilled as long as bimodal emulsions are not considered. The viscosity of the emulsion is a very important parameter to take into account. As the viscosity increases, stability also increases, but if the viscosity is too high it will affect the workability. If the viscosity is too high, problems will appear when pumping it from the storage tank to the manufacturing plant, and the wrapping with aggregates will not be correct. There are different procedures for preparing bitumen in water monomodal emulsions. The most widely used method in industrial processes is the one that uses colloid mills for its manufacture. This works with high temperatures, 140-170 \({}^{\circ}\)C, high pressures, 100-300 kPa, and/or high rotational speed, 5.000-10.000 rpm. In this process, emulsions are manufactured in a turbulent flow produced  
Figure 1: Storage stability evolution of an emulsion [8]. by the colloid mill rotating at high speed of around 105 s\({}^{-1}\). Emulsification generally takes place at concentrations of the dispersed phase (bitumen) between 60-70 wt. %, or at a bitumen volume fraction of 0.6-0.7. Above this concentration the process is not efficient, since very large drop particles are obtained and emulsions have no storage stability. Moreover, the droplet size obtained by the traditional method is between 5-10 microns; small droplet sizes are very difficult to obtain [18; 19]. Obviously, we could consider that the size of 5-10 microns is a small droplet size, but in this paper, we will further differentiate the necessity of different sizes. As we need a very small drop size to obtain a bimodal emulsion (around 1 \(\upmu\)m), we decided to use small droplet size for the emulsion (around 1 \(\upmu\)m) and large droplet size emulsions with 5-10 microns. Another manufacturing system used to disperse two immiscible phases is the HIPR procedure (high internal phase ratio) [20]. This procedure involves a direct blend of a highly viscous phase, 1-5000 Pa-s, with a second phase immiscible with the first one and in the presence of at least one surfactant. It works with low shear, 500-1500 rpm, in laminar flow and very little time, producing a viscoelastic paste which can be subsequently diluted to the required concentration of the dispersed phase. With this system, it is possible to obtain monomodal emulsions with very narrow particle size distribution and small mean droplet size (d(0.5)s, about 1 \(\upmu\)m). Furthermore, this system can produce concentrated and highly concentrated bitumen emulsions of 70-95 wt. %. The droplet size of the emulsions produced by this procedure can be easily controlled by rotational speed, formulation parameters, or by the concentration of the dispersed phase used during manufacture [20; 21]. Monomodal emulsions are characterized by one particle size distribution while bimodal emulsions are characterized by having two different and controlled droplet size and distributions. The optimal formulation contains a first small size of about 1 \(\upmu\)m and a second size of about 5 \(\upmu\)m with a proportion of \(\frac{1}{2}\), respectively (Figure 2) [22; 23; 24]. The decrease in viscosity that can be observed in bimodal emulsions when compared to their equivalent monomodal emulsions can be attributed to the packing of the emulsion droplets. Viscosity reduction can be attained for different kinds of fine and coarse emulsion associations [22].