On the injection depth of the laser-induced microjet into the soft material

Abstract

Recently, a laser-induced microjet is expected to be applied to the needle-free injection devices due to its unique highlyfocused jet shape. Considering practical use of the needle-free injection, we should control the injection depth. We investigate experimentally the effect of the distance L where a microjet travels before impacting at a soft material on injection depth D_i. We clarify the trend that, with increase of the distance L, the injection depth D_i increases, obtains a peak value, and then decreases. Furthermore, experimental results suggest that the focused jet shape is important for jet injection. Additionally, we consider what determines the distance L_peak at which the injection depth D_i obtains a peak value. Based on scaling argument using the focusing time scale Δ_tf estimated by the initial velocity U₀ of free surface and the curvature radius r of free surface, we find that the curvature radius r has a great influence on the distance L_peak. In order to verify this, we normalize the result of the injection experiments with the capillary tubes of various inner diameter d. The normalized result indicate that the distance L_peak can be described by the curvature radius r.