Uncovering transitional behavior of slug flow under different flow conditions constitutes a challenging problem of significant importance. Based on the experimental multi-channel measurements, we first calculate multivariate pseudo Wigner distribution (MPWD) and extract time-frequency features to probe the transient behavior of slug flow and churn flow. Then we calculate the multivariate multiscale sample entropy (MMSE) for different flow conditions and find that the MMSE allows us to uncover the dynamic flow behavior governing the transition from slug flow to churn flow. Our results indicate combining the MPWD and MMSE enables to reveal the transient and multiscale flow behavior underlying the formation and transition of slug flow and churn flow. (C) 2016 Elsevier B.V. All rights reserved.