CFD analysis of mucus bridge instability and breakup in the vocal folds

The atomization of airway mucus during speech represents a primary mechanism for respiratory aerosol generation, which is central to airborne pathogen transmission. While extensive research has examined single-phase phonation aerodynamics, the multiphase dynamics of droplet formation has received less attention. This study aims to fill this critical gap by presenting a multiphase computational fluid dynamics (CFD) model of the human vocal folds. The dynamic formation and subsequent breakup of the mucus bridge are simulated using a volume-of-fluid method coupled with an analytical model describing the time-dependent shape of the glottal orifice. The investigation reveals that initial, small-scale stochastic perturbations on the mucus surface are essential for triggering a physically realistic, multimodal breakup, in contrast to the delayed, symmetric rupture observed in idealized smooth-film simulations. During vocal-folds opening, the bridge thickness exhibits exponential decay rupturing after <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>2.32</a:mn> <a:mspace width="0.28em"/> <a:mtext>ms</a:mtext> </a:mrow> </a:math> at a dynamic aspect ratio of approximately 20, a value significantly exceeding the static stability limit. Furthermore, a parametric study reveals that increasing transglottal pressure from <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>491</c:mn> <c:mspace width="0.28em"/> <c:mtext>Pa</c:mtext> </c:mrow> </c:math> to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mn>1472</e:mn> <e:mspace width="0.28em"/> <e:mtext>Pa</e:mtext> </e:mrow> </e:math> , corresponding to louder phonation, accelerates the thinning process and leads to earlier initial rupture after <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mn>2.04</g:mn> <g:mspace width="0.28em"/> <g:mtext>ms</g:mtext> </g:mrow> </g:math> . This work provides fundamental insights into the precursors of respiratory aerosol generation