30 research outputs found
Trapping of drops by wetting defects
Controlling the motion of drops on solid surfaces is crucial in many natural phenomena and technological processes including the collection and removal of rain drops, cleaning technology and heat exchangers. Topographic and chemical heterogeneities on solid surfaces give rise to pinning forces that can capture and steer drops in desired directions. Here we determine general physical conditions required for capturing sliding drops on an inclined plane that is equipped with electrically tunable wetting defects. By mapping the drop dynamics on the one-dimensional motion of a point mass, we demonstrate that the trapping process is controlled by two dimensionless parameters, the trapping strength measured in units of the driving force and the ratio between a viscous and an inertial time scale. Complementary experiments involving superhydrophobic surfaces with wetting defects demonstrate the general applicability of the concept. Moreover, we show that electrically tunable defects can be used to guide sliding drops along actively switchable tracksâwith potential applications in microfluidic
RP105 deficiency attenuates early atherosclerosis via decreased monocyte influx in a CCR2 dependent manner
AbstractObjective: Toll like receptor 4 (TLR4) plays a key role in inflammation and previously it was established that TLR4 deficiency attenuates atherosclerosis. RadioProtective 105 (RP105) is a structural homolog of TLR4 and an important regulator of TLR4 signaling, suggesting that RP105 may also be an important effector in atherosclerosis. We thus aimed to determine the role of RP105 in atherosclerotic lesion development using RP105 deficient mice on an atherosclerotic background. Methods and results: Atherosclerosis was induced in Western-type diet fed low density lipoprotein receptor deficient (LDLrâ/â) and LDLr/RP105 double knockout (LDLrâ/â/RP105â/â) mice by means of perivascular carotid artery collar placement. Lesion size was significantly reduced by 58% in LDLrâ/â/RP105â/â mice, and moreover, plaque macrophage content was markedly reduced by 40%. In a model of acute peritonitis, monocyte influx was almost 3-fold reduced in LDLrâ/â/RP105â/â mice (P = 0.001), while neutrophil influx remained unaltered, suggestive of an altered migratory capacity of monocytes upon deletion of RP105. Interestingly, in vitro stimulation of monocytes with LPS induced a downregulation of CCR2, a chemokine receptor crucially involved in monocyte influx to atherosclerotic lesions, which was more pronounced in LDLrâ/â/RP105â/â monocytes as compared to LDLrâ/â monocytes. Conclusion: We here show that RP105 deficiency results in reduced early atherosclerotic plaque development with a marked decrease in lesional macrophage content, which may be due to disturbed migration of RP105 deficient monocytes resulting from CCR2 downregulation
Wettability-independent bouncing on flat surfaces mediated by thin air films
The impingement of drops onto solid surfaces1, 2 plays a crucial role in a variety of processes, including inkjet printing, fog harvesting, anti-icing, dropwise condensation and spray coating3, 4, 5, 6. Recent efforts in understanding and controlling drop impact behaviour focused on superhydrophobic surfaces with specific surface structures enabling drop bouncing with reduced contact time7, 8. Here, we report a different universal bouncing mechanism that occurs on both wetting and non-wetting flat surfaces for both high and low surface tension liquids. Using high-speed multiple-wavelength interferometry9, we show that this bouncing mechanism is based on the continuous presence of an air film for moderate drop impact velocities. This submicrometre âair cushionâ slows down the incoming drop and reverses its momentum. Viscous forces in the air film play a key role in this process: they provide transient stability of the air cushion against squeeze-out, mediate momentum transfer, and contribute a substantial part of the energy dissipation during bouncing
Bouncing on thin air; how squeeze forces in the air film during non-wetting droplet bouncing lead to momentum transfer and dissipation
Millimetre-sized droplets are able to bounce multiple times on flat solid substrates irrespective of their wettability, provided that a micrometre-thick air layer is sustained below the droplet, limiting We to âČ4. We study the energy conversion during a bounce series by analysing the droplet motion and its shape (decomposed into eigenmodes). Internal modes are excited during the bounce, yet the viscous dissipation associated with the in-flight oscillations accounts for less than 20 % of the total energy loss. This suggests a significant contribution from the bouncing process itself, despite the continuous presence of a lubricating air film below the droplet. To study the role of this air film we visualize it using reflection interference microscopy. We quantify its thickness (typically a few micrometres) with sub-millisecond time resolution and âŒ30 nm height resolution. Our measurements reveal strong asymmetry in the air film shape between the spreading and receding phases of the bouncing process. This asymmetry is crucial for effective momentum reversal of the droplet: lubrication theory shows that the dissipative force is repulsive throughout each bounce, even near lift-off, which leads to a high restitution coefficient. After multiple bounces the droplet eventually hovers on the air film, while continuously experiencing a lift force to sustain its weight. Only after a long time does the droplet finally wet the substrate. The observed bounce mechanism can be described with a single oscillation mode model that successfully captures the asymmetry of the air film evolution
Repeated bouncing of drops on wetting and non-wetting surfaces mediated by a persisting thin air film
Abstract Submitted
for the DFD14 Meeting of
The American Physical Society
Repeated bouncing of drops on wetting and non-wetting sur-
faces mediated by a persisting thin air lm JOLET DE RUITER, RUDY
LAGRAAUW, DIRK VAN DEN ENDE, FRIEDER MUGELE, MESA+ Institute
for Nanotechnology, University of Twente â Liquid drops impinging onto solid surfaces
undergo a variety of impact scenarios such as splashing, sticking, and bouncing,
depending on impact conditions and substrate properties. Bouncing requires efficient
conversion of initial kinetic energy into surface energy and back into kinetic
energy. This process is believed to be limited to non-wetting, in particular superhydrophobic
surfaces, for which viscous dissipation during drop-substrate contact
is minimal. Here, we report a novel bouncing mechanism that applies equally to
non-wetting and wetting systems for flat surfaces with contact angles down to 10
degrees. For initial impact speeds up to about 0.5 m/s we demonstrate using dual
wavelength interferometry that aqueous and non-aqueous drops remain separated
from the substrate by air films of (sub)micrometer thickness at all times throughout
a series of up to 16 consecutive bouncing events. We show that the purely dissipative
force arising from the viscous squeeze-out of air is responsible for both the
momentum transfer and for a substantial part of the residual energy dissipation.
Jolet de Ruiter
MESA+ Institute for Nanotechnology, University of Twente
Date submitted: 31 Jul 2014 Electronic form version 1.