What is used to mix substances together in cosmetic formulations?

The correct answer is emulsifier, as it plays a crucial role in cosmetic formulations by allowing the blending of two immiscible liquids, such as oil and water. Emulsifiers function by reducing surface tension, which enables the ingredients to mix evenly and remain stable over time. This is essential in many cosmetic products, including creams and lotions, where achieving a smooth, consistent mixture is important for efficacy and user experience.

The other options serve different functions in formulations: a binder is used to hold ingredients together and can affect texture but does not facilitate mixing; a solvent is typically used to dissolve other substances or to carry active ingredients but does not specifically aid in the mixing of immiscible substances; and a thickener increases the viscosity of a product but does not play a direct role in the process of mixing different types of ingredients. Understanding the distinct roles of these components is essential for developing effective cosmetic products.