  We present experimental observations and numerical simulations of a wrinkling
instability that occurs at sufficiently high strain rates in the trembling
regime of vesicle dynamics in steady linear flow. Spectral and statistical
analysis of the data shows similarities and differences with the wrinkling
instability observed earlier for vesicles in transient elongation flow. The
critical relevance of thermal fluctuations for this phenomenon is revealed by a
simple model using coupled Langevin equations that reproduces the experimental
observations quite well.
