arXiv subjects

Hiroya Watanabe

Publications and source records attributed to Hiroya Watanabe.

5 recordsLinked to original sources

arXiv2026

Penetration of impact-induced jets into skin-simulating materials

This study compares the penetration characteristics of impact-induced jets with those of laser-induced jets, focusing on the underlying penetration mechanism rather than device performance for needle-free injection. Using an impact-induced jet system capable of ejecting a highly focused liquid jet at high speed without the use of lasers, we examine jet penetration into skin-simulating materials. Unlike conventional needle-free injectors that produce diffused liquid jets, the impact-induced method generates a highly focused jet that limits the injected area, thereby reducing invasiveness. Comparative experiments with laser-induced jets show that, even at similar jet tip velocities, impact-induced jets achieve greater penetration depth. The penetration depth remains constant regardless of the offset distance D from the target, owing to the high and nearly uniform velocity of the cylindrical jet root region, indicating that penetration is governed by the cylindrical jet structure. Furthermore, we systematically vary the liquid viscosity, jet inertia, and elastic modulus of the skin-simulating material. To account for cylindrical liquid jet penetration, a shear deformation model is proposed, in which the jet kinetic energy is dissipated through deformation of the gelatin. The model shows good agreement with experimental results and provides a unified physical basis for liquid jet penetration.

physics.flu-dyn↗

arXiv2026

Interfacial dynamics induced by impacts across rigid and soft substrates

We investigate impact-induced gas-liquid interfacial dynamics through experiments in which a liquid-filled container impacts substrates with elastic moduli from $O(10^{-1})$ MPa to $O(10^{5})$ MPa. Upon impact, the concave gas-liquid interface inside the container deforms and emits a focused jet. When the jet velocity is normalized by the container impact velocity, all data collapse onto a single curve when plotted against the Cauchy number, $Ca = \rho_{\rm e} V_{\rm i}^2 / E$, which represents the ratio of the inertial force of the container-liquid system to the elastic restoring force of the substrate. The dimensionless jet velocity remains nearly constant for $Ca< 10^{-4}$, but decreases significantly for $Ca > 10^{-4}$. Based on this observation, we define the boundary between the rigid-impact and soft-impact regimes using the Cauchy number, providing a quantitative criterion for what constitutes ``softness'' in impact-driven interfacial flows. To explain the reduction in jet velocity observed in the soft-impact regime, we introduce a framework in which only the impulse transferred within the effective time window for jet formation contributes to interface acceleration. This concept, referred to as the partial impulse, captures the situation where the impact interval (the duration of contact between the container and the substrate) exceeds the focusing interval (the time required for jet formation). By modelling the contact force using an elastic foundation model and solving the resulting momentum equation over the finite impulse window, we quantitatively reproduce the experimental results. This partial impulse framework unifies the dynamics of impact-driven jetting across both rigid and soft substrate regimes, extending the applicability of classical impulse-based models.

cond-mat.soft↗

arXiv2026

Impulse-induced liquid jets from bubbles with arbitrary contact angles

This paper investigates the relationship between the contact angle of a spherical bubble attached to a tube submerged in a container and the jet speed induced by an impulsive acceleration at its base. While it has been well established that bubble geometry strongly influences the ejection speeds of liquid jets, mathematical studies of liquid jets with arbitrary bubble shapes remain limited. In this work, we derive a pressure impulse in the small-cavity limit as a tractable integral of classical Legendre functions. It is shown that the jet speed can be divided into two components: (i) the velocity induced by the hydrostatic pressure impulse distribution created by the curvature of the bubble, and (ii) the velocity induced by the distribution of the submersion of the tube in a container. This decomposition reveals that an optimal bubble curvature emerges only when the tube is submerged: the optimality is absent for non-submerged configurations, where the jet speed increases monotonically with bubble depth. Experiments confirm this non-monotonicity and quantitatively support the predicted shift of the optimal geometry with submersion depth.

physics.flu-dyn↗

arXiv2025

Effect of converging shape of container on the velocity of impact-induced focused liquid jet

We investigated the effect of container shape on the behavior of the impact-induced focused liquid jets by dropping a converging-shaped container (e.g., Kjeldahl flask) partially filled with liquid onto a floor to develop a method for increasing the jet velocity. Note that a similar well-known experiment, Pokrovski's experiment, in which a focused liquid jet is generated in a test tube, is free from the effect of the converging shape. The results showed that the jet was up to about 1.6 times faster in a converging-shaped container than in a test tube, despite the same impact velocity. To understand the mechanism of the increase in the jet velocity, the Laplace equation on the pressure impulse was solved numerically under the boundary condition that the pressure impulse is given at the bottom of the container. The normalized gas-liquid interfacial velocity obtained from the numerical solution of the pressure impulse field agrees well with the normalized jet velocity in experiments, showing that the jets we observed are driven by the pressure impulse generated at the bottom. In addition, numerical solutions of pressure impulse fields in a simpler-shaped container with different degrees of convergence were compared with analytical solutions obtained from a lower-order model of the pressure impulse field. We confirmed that the gas-liquid interfacial velocity of the impact-induced focused liquid jet is governed by changes in both the flow rate and pressure impulse gradient at the central axis of the container caused by changes in the cross-sectional area of the container. We showed that by changing the container shape, we can increase the velocity of the gas-liquid interface after the container impact. This finding is expected to be applied to the ejection and application of high-viscosity liquids as well as to needle-free injection technology using fast focused liquid jets.

physics.flu-dyn↗

arXiv2023

A phase diagram of the pinch-off behavior of impulsively-induced viscoelastic liquid jets

In this study, we systematically investigate the behaviors of viscoelastic liquid jets using an impulsive force, particularly in the high velocity and high elasticity regimes. The resulting jets are categorized into two types: (i) pinch-off jets, which break up during elongation after ejection, and (ii) no-pinch-off jets, which either retract to the nozzle after maximum elongation, known as `bungee-jumper jets' or return without elongation after ejection. We then propose criteria to delineate these regions using Reynolds number $Re$ and Weissenberg number $Wi$, reflecting the initial conditions at the jet ejection and the solution's rheological properties, respectively. We find that pinch-off jets occur at $Re \gtrsim 23.4Wi$ in high elasticity regimes ($Wi \gtrsim 10$), and at $Re \gtrsim 250$ in low elasticity regimes ($Wi \lesssim 10$). In addition, we demonstrate that the phase diagram of these behaviors can be rationalized through the focused jet modeling using the finitely extensible non-linear elastic dumbbell model with the Chilcott-Rallison closure approximation (FENE-CR).

physics.flu-dyn↗