44 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
On the spreading of impacting drops
The energy budget and dissipation mechanisms during droplet impact on solid
surfaces are studied numerically and theoretically. We find that for high
impact velocities and negligible surface friction at the solid surface (i.e.
free-slip), about one half of the initial kinetic energy is transformed into
surface energy, independent of the impact parameters and the detailed energy
loss mechanism(s). We argue that this seemingly universal rule is related to
the deformation mode of the droplet and is reminiscent of pipe flow undergoing
a sudden expansion, for which the head loss can be calculated by multiplying
the kinetic energy of the incoming flow by a geometrical factor. For impacts on
a no-slip surface also dissipation in the shear boundary layer at the solid
surface is important. In this case the geometric head loss acts as a lower
bound on the total dissipation (i.e. the spreading on a no-slip surface
approaches that on a free-slip surface when the droplet viscosity is send to
zero).
This new view on the impact problem allows for simple analytical estimates of
the maximum spreading diameter of impacting drops as a function of the impact
parameters and the properties of the solid surface. It bridges the gap between
previous momentum balance approaches and energy balance approaches, which
hitherto did not give consistent predictions in the low viscosity limit. Good
agreement is found between our models and experiments, both for impacts on
"slippery" or lubricated surfaces (e.g. Leidenfrost droplet impacts and head-on
droplet-droplet collisions) and for impacts on no-slip surfaces
Architected Polymer Foams Via Direct Bubble Writing
Polymer foams are cellular solids composed of solid and gas phases, whose mechanical, thermal, and acoustic properties are determined by the composition, volume fraction, and connectivity of both phases. A new high-throughput additive manufacturing method, referred to as direct bubble writing, for creating polymer foams with locally programmed bubble size, volume fraction, and connectivity is reported. Direct bubble writing relies on rapid generation and patterning of liquid shell–gas core droplets produced using a core–shell nozzle. The printed polymer foams are able to retain their overall shape, since the outer shell of these bubble droplets consist of a low-viscosity monomer that is rapidly polymerized during the printing process. The transition between open- and closed-cell foams is independently controlled by the gas used, while the foam can be tailored on-the-fly by adjusting the gas pressure used to produce the bubble droplets. As exemplars, homogeneous and graded polymer foams in several motifs, including 3D lattices, shells, and out-of-plane pillars are fabricated. Conductive composite foams with controlled stiffness for use as soft pressure sensors are also produced
Ejection Regimes in Picosecond Laser-Induced Forward Transfer of Metals
Laser-induced forward transfer (LIFT) is a 3D direct-write method suitable for precision printing of various materials, including pure metals. To understand the ejection mechanism and thereby improve deposition, here we present visualizations of ejection events at high-spatial (submicrometer) and high-temporal resolutions, for picosecond LIFT of copper and gold films with a thickness 50  nm≤d≤400  nm . For increasing fluences, these visualizations reveals the fluence threshold below which no ejection is observed, followed by the release of a metal cap (i.e., a hemisphere-shaped droplet), the formation of an elongated jet, and the release of a metal spray. For each ejection regime, the driving mechanisms are analyzed, aided by a two-temperature model. Cap ejection is driven by relaxation of thermal stresses induced by laser-induced heating, whereas jet and spray ejections are vapor driven (as the metal film is partly vaporized). We introduce energy balances that provide the ejection velocity in qualitative agreement with our velocity measurements. The threshold fluences separating the ejection regimes are determined. In addition, the fluence threshold below which no ejection is observed is quantitatively described using a balance between the surface energy and the inertia of the (locally melted) film. In conclusion, the ejection type can now be controlled, which allows for improved deposition of pure metal droplets and spray
Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer
3D printing of common metals is highly challenging because metals are generally solid at room conditions. Copper and gold pillars are manufactured with a resolution below 5 ÎĽm and a height up to 2 mm, using laser-induced forward transfer to create and eject liquid metal droplets. The solidified drop's shape is crucial for 3D printing and is discussed as a function of the laser energ
Highly focused supersonic microjets
The paper describes the production of thin, focused microjets with velocities
up to 850 m/s by the rapid vaporization of a small mass of liquid in an open
liquid-filled capillary. The vaporization is caused by the absorption of a
low-energy laser pulse. A likely explanation of the observed phenomenon is
based on the impingement of the shock wave caused by the nearly-instantaneous
vaporization on the free surface of the liquid. An experimental study of the
dependence of the jet velocity on several parameters is conducted, and a
semi-empirical relation for its prediction is developed. The coherence of the
jets, their high velocity and good reproducibility and controllability are
unique features of the system described. A possible application is to the
development of needle-free drug injection systems which are of great importance
for global health care.Comment: 10 pages, 11figure
Highly focused supersonic microjets
The paper describes the production of thin, focused microjets with velocities
up to 850 m/s by the rapid vaporization of a small mass of liquid in an open
liquid-filled capillary. The vaporization is caused by the absorption of a
low-energy laser pulse. A likely explanation of the observed phenomenon is
based on the impingement of the shock wave caused by the nearly-instantaneous
vaporization on the free surface of the liquid. An experimental study of the
dependence of the jet velocity on several parameters is conducted, and a
semi-empirical relation for its prediction is developed. The coherence of the
jets, their high velocity and good reproducibility and controllability are
unique features of the system described. A possible application is to the
development of needle-free drug injection systems which are of great importance
for global health care.Comment: 10 pages, 11figure
Drop Shaping by Laser-Pulse Impact
We show how the deposition of laser energy induces propulsion and strong
deformation of an absorbing liquid body. Combining high speed with stroboscopic
imaging, we observe that a millimeter-sized dyed water drop hit by a millijoule
nanosecond laser pulse propels forward at several meters per second and deforms
until it eventually fragments. The drop motion results from the recoil momentum
imparted at the drop surface by water vaporization. We measure the propulsion
speed and the time-deformation law of the drop, complemented by
boundary-integral simulations. The drop propulsion and shaping are explained in
terms of the laser-pulse energy, the drop size, and the liquid properties.
These findings are, for instance, crucial for the generation of extreme
ultraviolet light in nanolithography machines.Comment: Submitted as research article to Physical Review Applied, 6 pages
with 6 figure