76 research outputs found
Structure- and fluid-dynamics in piezo inkjet printheads
Inkjet printing is an important technology in color document production [133].\ud
The rapid development of inkjet technology started off around the late fifties.\ud
Since then, many inkjet devices have seen the light of day. In this overview, the attention is mainly restricted to the development towards the most important inkjet concepts of today, namely continuous, piezoelectric, and thermal inkjet
Lattice Boltzmann method to study the contraction of a viscous ligament
We employ a recently formulated axisymmetric version of the multiphase
Shan-Chen (SC) lattice Boltzmann method (LBM) [Srivastava et al, in preparation
(2013)] to simulate the contraction of a liquid ligament. We compare the
axisymmetric LBM simulation against the slender jet (SJ) approximation model
[T. Driessen and R. Jeurissen, IJCFD {\bf 25}, 333 (2011)]. We compare the
retraction dynamics of the tail-end of the liquid ligament from the LBM
simulation, the SJ model, Flow3D simulations and a simple model based on the
force balance (FB). We find good agreement between the theoretical prediction
(FB), the SJ model, and the LBM simulations
Stability of viscous long liquid filaments
We study the collapse of an axisymmetric liquid filament both analytically
and by means of a numerical model. The liquid filament, also known as ligament,
may either collapse stably into a single droplet or break up into multiple
droplets. The dynamics of the filament are governed by the viscosity and the
aspect ratio, and the initial perturbations of its surface. We find that the
instability of long viscous filaments can be completely explained by the
Rayleigh-Plateau instability, whereas a low viscous filament can also break up
due to end pinching. We analytically derive the transition between stable
collapse and breakup in the Ohnesorge number versus aspect ratio phase space.
Our result is confirmed by numerical simulations based on the slender jet
approximation and explains recent experimental findings by Castrejon-Pita et
al., PRL 108, 074506 (2012).Comment: 7 page
Evaporation-triggered segregation of sessile binary droplets
Droplet evaporation of multicomponent droplets is essential for various
physiochemical applications, e.g. in inkjet printing, spray cooling and
microfabrication. In this work, we observe and study phase segregation of an
evaporating sessile binary droplet, consisting of a mixture of water and a
surfactant-like liquid (1,2-hexanediol). The phase segregation (i.e., demixing)
leads to a reduced water evaporation rate of the droplet and eventually the
evaporation process ceases due to shielding of the water by the non-volatile
1,2-hexanediol. Visualizations of the flow field by particle image velocimetry
and numerical simulations reveal that the timescale of water evaporation at the
droplet rim is faster than that of the Marangoni flow, which originates from
the surface tension difference between water and 1,2-hexanediol, eventually
leading to segregation
Evaporation-Induced Crystallization of Surfactants in Sessile Multicomponent Droplets
Surfactants have been widely studied and used in controlling droplet
evaporation. In this work, we observe and study the crystallization of sodium
dodecyl sulfate (SDS) within an evaporating glycerol-water mixture droplet. The
crystallization is induced by the preferential evaporation of water, which
decreases the solubility of SDS in the mixture. As a consequence, the crystals
shield the droplet surface and cease the evaporation. The universality of the
evaporation characteristics for a range of droplet sizes is revealed by
applying a diffusion model, extended by Raoult's law. To describe the
nucleation and growth of the crystals, we employ the 2-dimensional
crystallization model of Weinberg [J. Non-Cryst. Solids 1991, 134, 116]. The
results of this model compare favorably to our experimental results. Our
findings may inspire the community to reconsider the role of high concentration
of surfactants in a multicomponent evaporation system
Rayleigh-Taylor instability by segregation in an evaporating multi-component microdroplet
The evaporation of multi-component droplets is relevant to various
applications but challenging to study due to the complex physicochemical
dynamics. Recently, Li (2018) reported evaporation-triggered segregation in
1,2-hexanediol-water binary droplets. In this present work, we added 0.5 wt%
silicone oil into the 1,2-hexanediol-water binary solution. This minute
silicone oil concentration dramatically modifies the evaporation process as it
triggers an early extraction of the 1,2-hexanediol from the mixture.
Surprisingly, we observe that the segregation of 1,2-hexanediol forms plumes,
rising up from the rim of the sessile droplet towards the apex during the
droplet evaporation. By orientating the droplet upside down, i.e., by studying
a pendant droplet, the absence of the plumes indicates that the flow structure
is induced by buoyancy, which drives a Rayleigh-Taylor instability (i.e.,
driven by density differences & gravitational acceleration). From micro-PIV
measurement, we further prove that the segregation of the non-volatile
component (1,2-hexanediol) hinders the evaporation near the contact line, which
leads to a suppression of the Marangoni flow in this region. Hence, on long
time scales, gravitational effects play the dominant role in the flow
structure, rather than Marangoni flows. We compare the measurement of the
evaporation rate with the diffusion model of Popov (2005), coupled with
Raoult's law and the activity coefficient. This comparison indeed confirms that
the silicone-oil-triggered segregation of the non-volatile 1,2-hexanediol
significantly delays the evaporation. With an extended diffusion model, in
which the influence of the segregation has been implemented, the evaporation
can be well described
Ring-shaped colloidal patterns on saline water films
Hypothesis: Electrostatically stabilised colloidal particles destabilise when brought into contact with cations causing the particles to aggregate in clusters. When a drop with stabilised colloidal partices is deposited on a liquid film containing cations the delicate balance between the fluid-mechanical and physicochemical properties of the system governs the spreading dynamics and formation of colloidal particle clusters. Experiments: High-speed imaging and digital holographic microscopy were used to characterise the spreading process. Findings: We reveal that a spreading colloidal drop evolves into a ring-shaped pattern after it is deposited on a thin saline water film. Clustered colloidal particles aggregate into larger trapezoidally-shaped ‘supraclusters’. Using a simple model we show that the trapezoidal shape of the supraclusters is determined by the transition from inertial spreading dynamics to Marangoni flow. These results may be of interest to applications such as wet-on-wet inkjet printing, where particle destabilisation and hydrodynamic flow coexist.</p
Numerical simulations of the full ink-jet printing processes: From jetting to evaporation
Ink-jet printing requires to perfectly control both the jetting of droplets and the subsequent droplet evaporation and absorption dynamics. Considerable complexity arises due to the fact that ink is constituted of a mixture of different liquids, surfactants and pigments. Using a sharp-interface ALE finite element method, we numerically investigate the main aspects of ink-jet printing, both on the jetting side and on the drying side. We show how a short pause in jetting can result in clogged nozzles due to solvent evaporation and discuss approaches how to prevent this undesired phenomenon. Once the droplets have been jetted on paper and is evaporating, the print quality can be deteriorated by the well-known coffee-stain effect, i.e. the preferential deposition of particles near the rim of the droplet. This can be prevented in several ways, e.g. employing controlled Marangoni flow via surfactants or co-solvents or printing on a primer layer jetted in beforehand, thus creating a homogeneous deposition pattern for a perfect final printout
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