Robots designed for social interaction often express their internal and emotional states through nonverbal behavior. In most systems, this is achieved using facial expressions, gestures, locomotion, and tone of voice. In this paper, we propose a new expressive nonverbal channel for social robots in the form of a texture-changing skin. Our approach is inspired by biological systems, which frequently display their internal states through skin texture change. We present two designs for soft fluidic Texture Units (TU): goosebumps and spikes, and a design for an interleaved TU array that can function as an expressive social robot skin. We discuss fabrication, actuation, and control considerations, as well as a framework for mapping emotional states to texture changes. We exemplify the design in a social robot which has both a screen-projected face and the proposed texture-changing expressive skin.