chapter  9
56 Pages

9

I. Introduction ................................................................ 420 II. Viscoelastic Surfactant Solutions.............................. 421

A. Dilute Solutions of Spherical Micelles............... 421 B. Viscoelastic Solutions of Rod-Shaped Micelles . 423 C. Flow Properties of Surfactant Mesophases....... 425

III. Influence of Micellar Kinetics on Relaxation Properties of Entangled Solutions of Rod-Shaped Micelles ....................................................................... 427 A. Short Reminder on Micellar Kinetics ................ 427 B. Relaxation Properties of Entangled Solutions

of Rodlike Micelles .............................................. 428 IV. Linear Viscoelastic Properties of Entangled

Solutions of Rod-Shaped Micelles ............................. 435 A. Linear Viscoelastic Regime................................. 435 B. Methods of Investigation .................................... 435 C. Maxwell Model for Viscoelastic Solutions ......... 438 D. Experimental Results.......................................... 440

V. Nonlinear Viscoelastic Properties of Entangled Solutions of Rod-Shaped Micelles ............................. 444 A. Analytical Solutions for Steady-State

Shear Flow........................................................... 445 B. Transient Flow .................................................... 449

VI. Shear-Induced Phase Transitions ............................. 451 VII. Instabilities of Flow (Shear Banding) ...................... 456 VIII. Conclusions and Prospects ........................................ 459 References............................................................................ 460

I. INTRODUCTION

“panta rei” — All things are in a state of flux and as a function of time everything tends to flow; everything changes. This famous statement of the ancient Greek philosopher Heraclitus is still valid and expresses the basic ideas of rheological research. Many substances in daily life cannot be simply classified as solids or liquids, but show more complicated intermediate properties. Such systems are often composed of super molecular, colloidal microstructures. For many industrial applications, it is of special interest to explore the basic relationships between molecular structures of complex fluids and their mechanical properties. Such experience is valuable in the optimal design of products, including foams, emulsions, suspensions, gels, cosmetics, or cleaning liquids. Surfactant solutions are usually pumped, extruded, stirred, or mixed during their processing. Typical products based on advanced rheological techniques are drag-reducing liquids, which are used to transport fluids through elongated tubes or pipelines at relatively low energy costs. Shampoos or hair conditioners should exhibit gel-like properties in order to form stable foams. Emulsions or suspensions often have yield values, which will protect these systems against sedimentation or coagulation. Well-defined flow properties are often required in industry and research, and consequently there are increasing demands to study these phenomena. This chapter is an attempt to summarize recent knowledge on complex flow processes, with special emphasis drawn on nonlinear phenomena. The materials, as we will discuss in the next paragraphs,

are viscoelastic surfactant solutions. It turns out that many rheological properties of these solutions are controlled by micellar kinetics. Given this special situation, viscoelastic surfactant solutions can sometimes be characterized by simple theoretical laws. This holds, at least, in certain concentration regimes or certain temperature intervals. These special features lead to ideal conditions, which are difficult to realize in other types of colloidal systems. Viscoelastic surfactant solutions may, therefore, be used as simple model liquids in order to gain a deeper insight into fundamental principles of flow.