## 1 Introduction  
Transportation of liquids in pipelines and in the petroleum industries requires a high percentage of energy consumption. In several cases, energy is dissipated because fluids are conveyed at high turbulent flow, thereby resulting in frictional resistance called drag. In other cases, pumps should be used at strategic pumping stations to maintain the flow of these liquids. In view of these circumstances, measures are necessary to reduce energy dissipation, turbulence in the pipeline, and maintain the flow rate. Notable among these approaches is the use of polymeric additives, surfactants, fibers, and other drag reducing agents (DRAs). The frictional drag forces of fluids under turbulent flow conditions can be decreased significantly by adding a minute quantity of drag-reducing additives, such as polymers or surfactants. This approach is the most commonly applied technique to reduce the turbulent friction between a solid surface and fluid [1]. A maximum drag reduction (DR) percentage of approximately 80% in the pipe flow and approximately 50% in the rotating disk apparatus (RDA) can be easily obtained [2]. Different types of synthetic and natural polymers were initially used as drag-reducing additives for turbulent flow. Among these polymers are polyethylene oxide (PEO) [3; 4; 5; 6; 7], polyacrylamide (PAAM) [8; 9; 10], polyisobutylene (PIB) [11; 12; 13; 14], xanthan gum [15; 16; 17], and guar gum [18; 19], among others. However, most of these polymers in the solution are often influenced by mechanical degradation, which reduces their short-term DR effectiveness [19; 20]. Other materials that have the capability to withstand mechanical degradation by the formation of micelles have been explored as well. These materials are called surfactants and research has been conducted on these materials to investigate their DR ability [21; 22; 23]. However, surfactants are less effective than polymers [24]; thus, alternative solutions are necessary. Therefore, different surfactants are combined with these polymers to improve the latter's DR effectiveness and mechanical degradation resistance. The properties of solutions containing polymer and surfactant have been explained by assuming the formation of a polymer-surfactant complex resulting from the binding of the surfactant ions onto the polymer chain. These complex properties are extremely different than the properties of their individual materials, and rheological properties of solutions appear to have changed [25]. The interactions between polymers and surfactants have technical and fundamental importance and is extensively used in different applications, such as petroleum industry, pharmaceutical, and personal care products [26; 27]. Several parameters, such as polymer hydrophobicity, charge density, surfactant head group structure, ionic strength, and flexibility, are affected by the binding between the surfactants and polymers [28]. In synthetic polymer-surfactant systems, polyelectrolytes and oppositely charged surfactants produce the strongest combination [29]. Different complex mixtures have been used recently as DRAs in pipelines [24; 30; 31] and rotating disk apparatus [32; 33; 34; 35]. [PERSON] et al. [24] studied the interaction between an anionic copolymer, which comprises acrylamide and sodium acrylate (referred to as PAM) and cationic surfactant of octadecyltrimethylammonium chloride (OTAC), in pipeline flow. Deionized and tap water were used to prepare the complex solutions. The aforementioned authors observed that the polymer properties were significantly affected by surfactant addition, where the DR efficiency of PAM was decreased with the addition of OTAC. However, the influence of OTAC on the DR performance was considerably pronounced at low concentrations of PAM. [PERSON] et al. [34] analyzed the complex mixture of polyacrylamide (PAA) and sodium dodecyl sulfate (SDS) as DRA under turbulent flow in a rotating disk apparatus.