|Alternative Title||Mechanisms of colloidal particle deposition in an evaporating droplet and the experimental research in space|
|Place of Conferral||北京|
|Keyword||胶体液滴 蒸发 颗粒沉积 图案化表面 空间微重力|
The phenomenon of droplet evaporation occurs commonly in nature. Over the last two decades, there has been a surge of scientific interest in the microflow and deposition of particles in an evaporating droplet. This is not only because the evaporation process involves multiple physical phenomenon such as the common coffee ring effect, but also because these phenomena influence a wide range of applications such as microfluidic, photonic crystal formation, disease diagnosis, etc. However, the self-assembling dynamic theory of particles in drying droplet is still incomplete, which has greatly limited its application.
In this doctoral dissertation, I have participated in the design of the colloidal material box that is a subsystem and payload of SJ-10 satellite. The payload is composed of five parts: a structure unit, an injection management unit, a sample management unit, an optical observation unit, and a drive control unit. Its main functions include uniform dispersion, management and control, heating and evaporation, and optical observation of complex fluids in space. We have further launched a lot of ground testing and test validation, which show that the colloid material box could satisfy the demands of both droplet evaporation and colloidal assembling experiments in space microgravity environment. We have successfully completed the in-orbit experiments in space.
Based on space experimental platform (colloid material box), we have carried out the drop manipulation experiment in space successfully by using the hydrophilic/hydrophobic patterned surface technique, and the mechanism of the drop wetting and control on the patterned substrate during drop evaporation have been further studied. The results show that the wetting behavior of the patterned substrate has three regimes: the constant hydrophobic angle mode, the constant contact area mode and the constant hydrophilic angle mode. The contact area regime is responsible for the drop control. The drop shape in normal gravity and microgravity can be described as ellipsoidal cap and spherical cap model, and the patterned substrate could confine aqueous drops with larger volume under microgravity than in normal gravity. With advantages of simple operation and strong capability to control large drops, this technique exhibite the wide application prospect in the fields of fluid management, bio-sensing, pharmacy in microgravity condition in the future.
The rupture of liquid film and rearrangement of particles in the final stage of droplet evaporation has been investigated by utilizing differential interference contrast microscopy. We have revealed the formation mechanism of a network pattern inside a coffee ring, which is consist of a large number of dry patches. Real-time bottom-view images show that the evolution of a typical dry patch can be divided into three stages: rupture initiation, dry patch formation and expansion, and drying of the residual liquid. It could be shown that particles aggregation and liquid film dewetting are mutual influenced by each other. The uneven distribution of particles lead to the suction effect, which promotes the rupture of the liquid film and the formation of the dry patch at the particle-poor region; the particle-assembling process is totally controlled by the liquid film dewetting and dominated by the surface tension of the liquid film, which eventually determine the ultimate deposition patterns. This work expands the coffee ring effect proposed by Deegan, and enriches the theories of particles deposition in an evaporating droplet.
Experiments of colloidal droplet evaporation has been conducted in both space microgravity and terrestrial normal gravity conditions. By analysis of the profile change and particles motion in an evaporating droplet, we have illuminated the gravity effect on the particles deposition process and the mechanism of the weakened coffee ring effect. We report on the interplay of the interface shrinkage, the gravitational sedimentation and the outward capillary flow in drying droplets. This interaction effect is the inference we draw from deposition patterns of both sessile and pendant droplets, which contain particles in different sizes, evaporating on a patterned substrate. We have proposed two different regimes for the relative motion between the particles and the interface: the pursuit regime (sessile droplet) and the meeting regime (pendant droplet), which further the theory of particles deposition in drying droplet. It is significant to predict and regulate the morphology of the final deposition patterns.
This thesis has finally conducted a preliminary exploration research of the deposition patterns control based on the theories of particle deposition in an evaporating droplet. Through the base heating, we could increase the efficiency of the colloidal crystal preparation and obtain high quality closed packed structure. By controlling the initial droplet volume, particle concentration and particle sizes, the deposition morphology can be regulated. Using the method of surface modification and specific physical effects in an evaporating droplet, the deposition patterns near the edge and the center of the droplet have been modified. By using the interplay of the interface shrinkage, the gravitational sedimentation and the outward capillary flow in a drying pendant droplet, we have successfully realized the separation of colloidal particles with different sizes. The manipulation of deposition patterns will help to develop new functional microstructures and the related materials in the future.
|李伟斌. 蒸发液滴中的胶体颗粒沉积机制与空间实验研究[D]. 北京. 中国科学院大学,2018.|
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