Cavitation in trees: the dynamics and sound of bubble nucleation
The sap within trees circulates in tiny “microfluidic” wood vessels. Under hydric stress, in dry weather conditions, the sap can cavitate. The origin of cavitation is that water can achieve negative pressures (tension), because of evaporation at the leaves. Here, we focus on the dynamics of the cavitation bubble, which is of primary importance to understand the resistance of trees to cavitation. First we use the method of artificial trees, building stiff transparent hydrogels to mimick wood channels. Our experiments on tensed water confined in micrometric voids show an extremely fast dynamics: bubbles are nucleated within a microsecond timescale. The bubble pulsates with transient oscillations at very high frequencies in the MHz range, much higher than Minnaert’s frequency for unconfined bubbles. This rich dynamics can be accounted for by a model we developed, leading to a modified Rayleigh-Plesset equation, accounting for the confinement. These oscillations may be at the origin of the short acoustic emissions that are recorded in real trees under hydric stress. We will end up with experimental results on real wood slices, combining optical and acoustical recordings. We show that the bubble ultrasound amplitude depends on the wood channel size.