Abstract
We experimentally study the displacement of viscous liquid by gas in a square capillary tube. The liquid is partially wetting in a way that no spontaneous imbibition along the interior corners would occur even in the absence of forced displacement. The gas–liquid interface exhibits a variety of morphologies with an increasing displacement rate. At a low displacement rate, a constantly moving meniscus can be observed, without any liquid deposition on the tube wall. An increase in the displacement rate gives rise to the deposition of two ultra-thin liquid filaments at each corner, which immediately break into tiny droplets. An additional thicker filament is entrained at each corner as the displacement rate further increases, connecting the thinner ones and the meniscus. When the displacement rate is high, liquid films are entrained on the tube wall and eventually collapse, entrapping an amount of gas in the form of Taylor bubbles. Quantitative measurements show that both the thicker filaments and the liquid films retract at constant speeds. Empirical relations predicting the film thickness and the bubble length are proposed and agree with the experimental results.
Highlights
The displacement of two immiscible fluids in capillary channels plays an important role in nature and industry, such as water infiltration into the soil,1 inkjet printing,2 microchannels of central processing unit (CPU) cooling devices,3 microanalytical devices,4 and so on
We explored the relationship between the thickness of the entrained liquid film and the capillary number
The main governing parameter of the problem is the capillary number Ca = μU/σ, which can be regarded as the dimensionless displacement speed
Summary
The displacement of two immiscible fluids in capillary channels plays an important role in nature and industry, such as water infiltration into the soil, inkjet printing, microchannels of central processing unit (CPU) cooling devices, microanalytical devices, and so on. In these displacement processes, an inevitable challenge is the movement of the contact line, where the fluid interface intersects the wall.. The plate is withdrawn from a reservoir of viscous liquid at a certain speed U. The governing parameter is the dimensionless speed, i.e., the capillary number Ca = μU/σ, where μ is the dynamic viscosity of the liquid and σ is the surface tension coefficient. There exists a threshold of Ca to distinguish different interface morphologies from moving patterns of the contact line.
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