Introduction

This book’s title requires clarifying two concepts: “tip streaming” and “complex fluid.” In essence, tip streaming is the flow producing tiny drops or jets from the tip of a mother droplet, fluid meniscus, or liquid film. We devote the first part of this chapter to motivating and contextualizing the tip streaming phenomenon. We also introduce the microfluidic configurations commonly used to produce this singular flow. These configurations and their specific applications will be analyzed in more detail throughout the text. The term “complex fluids” has been coined to refer to substances involving the coexistence of several phases, such as foams, suspensions, and emulsions. It also refers to fluids containing colloids and macromolecules, as well as liquid crystals and quantum fluids, among others. Here, we use this term to refer only to liquids containing surfactant molecules or polymer chains, which produce interfacial and bulk viscoelastic effects, respectively. As explained in other chapters, surfactants and polymers substantially alter the tip streaming flow in many technologically relevant applications. In the last part of this chapter, we briefly describe these two elements. We also introduce the effect of externally applied electric fields, which drive the flow in many tip streaming realizations.