Enhancing the electric
field strength
can facilitate the approach of droplets and the drainage of liquid
film. However, two droplets do not coalesce but bounce off after contact
under an excessively high electric field strength. To reveal the underlying
mechanism, the dynamic behaviors of two free droplets suspended in
low-viscosity silicone oil under a DC electric field were investigated
herein. Three distinct behavior modes were successively observed by
a high-speed camera with the increase in electric field strength:
coalescence, partial coalescence, and noncoalescence. The mechanisms
and key criteria of partial coalescence and noncoalescence were explored
by studying the competition between electric force and interfacial
force. The theoretical formula of critical electric field strength
for droplet coalescence was derived and validated by experiments.
The results indicated that the electric capillary number Ca can be
used as the criterion to identify the behavior modes of two free droplets.
The droplets undergo partial coalescence or noncoalescence when Ca >
0.11; otherwise, the droplets experience coalescence.
The liquid entrainment in Higee devices exists and reduces separation efficiency. The flooding in Higee devices, called Higee flooding, sets an upper limit of the allowable throughputs, and the different flooding criteria were proposed by researchers. Using air-water system, the experiments of liquid entrainment and Higee flooding were carried out in a rotating zigzag bed(RZB), as a new Higee device. The RZB rotor had an inner diameter of 0.101m, outer diameter of 0.284m and height of 0.051m. The distillation experiment was conducted in the same RZB to investigate the effect of the liquid entrainment on the separation efficiency. The results show that the liquid entrainment in RZB contains liquid-droplet and liquid-film entrainment. The liquid-film entrainment is far higher than the liquid-droplet entrainment. And the excessive liquid-film entrainment at the outer edge of RZB rotor severely lowered separation efficiency. The point, at which the pressure drop increases sharply with increasing superficial liquid velocity, was first to be taken as a new Higee flooding criterion. The new Higee flooding criterion reflected the real flooding in RZB, and the previous criteria could not reflect. The new Higee flooding criterion can also applied in rotating packed beds.
Coalescence of conducting
droplets dispersed in an immiscible medium
can be facilitated by an electric field. However, droplets recoil
promptly after contact in sufficiently high electric fields if the
cone angle between droplets exceeds a critical value. To elucidate
the critical condition for droplet coalescence, the behavior of two
suspended droplets after contact with a direct current electric field
is studied. It is shown that the critical angle is determined not
only by the droplet geometry but also conductivity, surfactant concentration,
and size. As the droplet conductivity increases, more identical ions
accumulate on the adjacent interfaces of two droplets due to the faster
ionic migration, which results in Coulombic repulsion between droplets
and a reduced critical angle. For surfactant-laden droplets, film
drainage induces a surfactant concentration gradient on the leading
edges of droplets, and then Marangoni stress is formed to reduce the
critical angle. In the case of large droplets, the bridge transiently
expands under the action of directional flow caused by further droplet
deformation, but eventually breaks due to opposite electrostatic forces.
Based on this finding, the electrocoalescence criterion can be determined
and employed to facilitate droplet coalescence in various applications.
Background
The immune cell compartment of the mammalian brain changes dramatically and peripheral T cells infiltrate the brain parenchyma during normal aging. However, the mechanisms underlying age-related T cell infiltration in the central nervous system remain unclear.
Results
Chronic inflammation and peripheral T cell infiltration were observed in the subventricular zone of aged mice. Cell-cell interaction analysis revealed that aged microglia released CCL3 to recruit peripheral CD8+ memory T cells. Moreover, the aged microglia shifted towards a pro-inflammation state and released TNF-α to upregulate the expression of VCAM1 and ICAM1 in brain venous endothelial cells, which promoted the transendothelial migration of peripheral T cells. In vitro experiment reveals that human microglia would also transit to a chemotactic phenotype when treated with CSF from the elderly.
Conclusions
Our research demonstrated that microglia play an important role in the aging process of brain by shifting towards a pro-inflammation and chemotactic state. Aged microglia promote T cell infiltration by releasing chemokines and upregulating adhesion molecules on venous brain endothelial cells.
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