62 research outputs found
Microdroplet impact at very high velocity
Water microdroplet impact at velocities up to 100 m/s for droplet diameters
from 12 to 100 um is studied. This parameter range covers the transition from
capillary-limited to viscosity-limited spreading of the impacting droplet.
Splashing is absent for all measurements; the droplets always gently spread
over the surface. The maximum spreading radius is compared to several existing
models. The model by Pasandideh-Fard et al. agrees well with the measured data,
indicating the importance of a thin boundary layer just above the surface, in
which most of the viscous dissipation in the spreading droplet takes place. As
explained by the initial air layer under the impacting droplet, a contact angle
of 180 degrees is used as model input
Needle-free injection into skin and soft matter with highly focused microjets
The development of needle-free drug injection systems is of great importance
to global healthcare. However, in spite of its great potential and research
history over many decades, these systems are not commonly used. One of the main
problems is that existing methods use diffusive jets, which result in scattered
penetration and severe deceleration of the jets, causing frequent pain and
insufficient penetration. Another longstanding challenge is the development of
accurate small volume injections. In this paper we employ a novel method of
needle-free drug injection, using highly-focused high speed microjets, which
aims to solve these challenges. We experimentally demonstrate that these unique
jets are able to penetrate human skin: the focused nature of these microjets
creates an injection spot smaller than a mosquito's proboscis and guarantees a
high percentage of the liquid being injected. The liquid substances can be
delivered to a much larger depth than conventional methods, and create a
well-controlled dispersion pattern. Thanks to the excellent controllability of
the microjet, small volume injections become feasible. Furthermore, the
penetration dynamics is studied through experiments performed on gelatin
mixtures (human soft tissue equivalent) and human skin, agreeing well with a
viscous stress model which we develop. This model predicts the depth of the
penetration into both human skin and soft tissue. The results presented here
take needle-free injections a step closer to widespread use
Wall forces on a sphere in a rotating liquid-filled cylinder
We experimentally study the behavior of a particle slightly denser than the
surrounding liquid in solid body rotating flow. Earlier work revealed that a
heavy particle has an unstable equilibrium point in unbounded rotation flows.
In the confinement of the rotational flow by a cylindrical wall a heavy sphere
with density 1.05 g/cm describes an orbital motion in our experiments. This
is due to the effect of the wall near the sphere, i.e. a repulsive force
(). We model on the sphere as a function of the distance from the
wall (): as proposed by Takemura and Magnaudet (2003).
Remarkably, the path from the model including reproduce the
experimentally measured trajectory. In addition during an orbital motion the
particle does not spin around its axis, and we provide a possible explanation
for this phenomenon.Comment: 11 pages, 11 figure
High-speed photoelastic tomography for axisymmetric stress fields in a soft material: temporal evolution of all stress components
This study presents a novel approach for reconstructing all stress components
of the dynamic axisymmetric fields of a soft material using photoelastic
tomography (PT) and a high-speed polarization camera. This study focuses on
static and dynamic Hertzian contact as an example of transient stress field
reconstructions. For the static Hertzian contact (a solid sphere pressed
against a gel block), all stress components in the urethane gel, which has an
elastic modulus of 47.4 kPa, were reconstructed by PT using the measured
photoelastic parameters. The results were compared with theoretical solutions
and showed good agreement. For the dynamic Hertzian contact (a sphere impacting
gel), a high-speed polarization camera was used to reconstruct the transient
stress field within the gel. PT was used to quantitatively measure the shear
and axial stress waves and showed different propagation speeds on the
substrate. The technique allowed the simultaneous measurement of stress fields
ranging from to kPa during large deformations,
demonstrating its accuracy in capturing rapidly changing stress tensor
components in dynamic scenarios. The scaling laws of the calculated impact
force agreed with theoretical predictions, validating the accuracy of PT for
measuring dynamic axisymmetric stress fields in soft materials.Comment: 16 pages, 15 figure
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