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.
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