Interfacial Evolution and Breakup Modes of Confined Fluid Threads in a Sudden Contraction Channel

The breakup of confined fluid threads in a circular sudden contraction microchannel was investigated by combining high-speed imaging with three-dimensional simulations using the Volume of Fluid (VOF) method. The effects of the capillary number Ca, normalized thread length L¯=L/(2R), lateral continuous phase flow rate Q3, and rear wall contact on interfacial evolution and breakup were examined. At Ca=O(10−5), representative experiments with Q1=0.60 mL/h and Q2=1.70 mL/h produced no breakup, rear-free breakup, rear-confined breakup, and multiple breakup at Q3=0.30, 0.58, 0.80, and 1.00 mL/h, respectively. These values represent operating conditions for the observed modes rather than universal transition thresholds. At Ca=O(10−4), the late-stage normalized curvature radii followed distinct fitted power laws. For rear-free breakup, R¯1∼τ¯0.257 and R¯m∼τ¯0.182, whereas for rear-confined breakup, R¯1∼τ¯0.350 and R¯m∼τ¯0.307. In the numerical simulations at a fixed mean fluid thread velocity of 9.23×10−4 m/s and L¯=1.3, increasing Q3 from 0.78 to 3.56 and 6.25 mL/h produced no breakup, rear-free breakup, and rear-confined breakup, respectively. The simulations reproduced the principal interfacial topology and event sequence observed experimentally and provided the three dimensional velocity and mean curvature fields. These results show that breakup is governed by the coupled effects of contraction geometry, external flow, thread length, and rear wall contact.