State classification and near-threshold dynamics of droplet detachment and stable levitation induced by an ultrasonic phased array on surfaces of different wettability

Wall-borne droplet levitation under ultrasound is a fundamental process in contactless droplet transport, yet it is still commonly described using a single detachment threshold. Here, a 40 kHz, 16×16 ultrasonic phased-array platform is used to examine how surface wettability affects droplet detachment and the experimentally observed stable-levitation window, including its lower and upper state boundaries. Three reproducible states—Attached, Stable levitation, and Breakup—are identified, and the finite stable-levitation window is defined by the levitation and breakup thresholds. Under the present wall-attached initialization and ground-condition actuation, stable noncontact levitation is sustained over a droplet-volume range of 1–25 μl. To compare different droplet sizes and wettability conditions on a common basis, we introduce the Bond number, Bo=ρgR2/σ, and a dimensionless acoustic–gravity index, I*=(U/Uref)2(Rref/R), where U is the applied driving voltage and Uref is the levitation-threshold voltage under the reference condition of CA = 170° and V=25 μl. Decreasing contact angle markedly shifts the levitation threshold upward and progressively narrows the stable-levitation window; the window disappears near Bo≈0.44, 0.38, and 0.19 for CA=170°, 164°, and 157°, respectively, and is absent over the tested range for CA=122° and 110°. Near-threshold dynamics further show that increasing relative overdrive raises the levitation height but also amplifies residual oscillations. These results demonstrate that acoustically induced levitation of wall-borne droplets is governed by a wettability-controlled operating window rather than by a single detachment threshold.