  As wearable devices move toward the face (i.e. smart earbuds, glasses), there
is an increasing need to facilitate intuitive interactions with these devices.
Current sensing techniques can already detect many mouth-based gestures;
however, users' preferences of these gestures are not fully understood. In this
paper, we investigate the design space and usability of mouth-based
microgestures. We first conducted brainstorming sessions (N=16) and compiled an
extensive set of 86 user-defined gestures. Then, with an online survey (N=50),
we assessed the physical and mental demand of our gesture set and identified a
subset of 14 gestures that can be performed easily and naturally. Finally, we
conducted a remote Wizard-of-Oz usability study (N=11) mapping gestures to
various daily smartphone operations under a sitting and walking context. From
these studies, we develop a taxonomy for mouth gestures, finalize a practical
gesture set for common applications, and provide design guidelines for future
mouth-based gesture interactions.
