Abstract
The airway surface liquid (ASL) is a very thin mucus layer and covers the vocal fold (VF) surface. Adhesion mediated by the ASL occurs during phonation as the VFs separate after collision. Such adhesion is hypothesized to determine voice quality and health. However, biomechanical insights into the adhesive processes during VF oscillation are lacking. Here, a computational study is reported on self-sustained VF vibration involving contact and adhesion. The VF structural model and the glottal airflow are considered fully three-dimensional. The mechanical behavior of the ASL is described through a constitutive traction–separation law where mucosal cohesive strength, cohesive energy, and rupture length enter. Cohesive energy values considered are bound below by the cohesive energy of water at standard temperature and pressure. Cohesive strength values considered are bound above by prior reported data on the adhesive strength of mucosal surface of rat small intestine. This model introduces a mechanical length scale into the analysis. The sensitivity of various aspects of VF dynamics such as flow-declination rate, VF separation under adhesive condition, and formation of multiple local fluid bridges is determined in relation to specific ASL adhesive properties. It is found that for the ASL considered here, the characteristics of the VF separation process are of debond type. Instabilities lead to the breakup of the bond area into several smaller bond patches. Such finding is consistent with in vivo observations.
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Abbreviations
- \(AB\) :
-
Line oriented in anterior–posterior direction and situated on medial plane
- \(C_L,C_R\) :
-
Collision-prone parts of \(S_L\) and \(S_R\)
- \(c_R\) :
-
Rayleigh wave speed in VF tissue
- \(d_g\) :
-
Initial distance between VFs
- \(d_p\) :
-
Fixed separation between planes \(P_L\) and \(P_R\)
- \(D\) :
-
VF medial–lateral extent (depth)
- \(\delta {\varvec{D}}_v\) :
-
Virtual strain tensor associated with \(\delta {\varvec{u}}_v\)
- \(\epsilon \) :
-
Volumetric strain
- \({\varvec{e}}\) :
-
Deviatoric strain tensor
- \(E\) :
-
Elastic modulus of VF tissue
- \(f\) :
-
Vibration frequency
- \(g_1\) :
-
Viscoelastic shear-modulus relaxation factor
- \(G\) :
-
VF tissue shear modulus
- \({\varvec{I}}\) :
-
Second-order identity tensor
- \(k_1\) :
-
Viscoelastic bulk-modulus relaxation factor
- \(K\) :
-
VF tissue bulk modulus
- \(l_c\) :
-
Total length of all interaction line segments
- \(l_{c,\mathrm{max}}\) :
-
Maximum value of \(l_c\) over a cycle
- \(L\) :
-
VF anterior–posterior extent (length)
- \(\hat{n}\) :
-
Unit vector directed normal to surface
- \(p\) :
-
Flow pressure
- \(p_\mathrm{in}\) :
-
Time-dependent flow pressure at glottal air tract inlet
- \(p_\mathrm{max}\) :
-
Maximum flow pressure at glottal air tract inlet
- \(P_L,P_R\) :
-
Rigid planes
- \(\hat{r}\) :
-
Arbitrary unit vector parallel to surface
- \(S_L,S_R\) :
-
Glottal surfaces; flow–structure interaction surfaces
- \(t\) :
-
Time
- \(\varDelta t\) :
-
Time increment
- \(t_c, t_o, t_e, t_f\) :
-
Dimensional times denoting local events related to ASL
- \(t_{c,\mathrm{min}}\) :
-
Time instant corresponding to the initiation of contact along line \(AB\)
- \(t_{\mathrm{cycle}}\) :
-
Time instant corresponding to the end of collision cycle
- \(t_{\mathrm{ramp}}\) :
-
Time duration for inlet pressure ramp
- \(\hat{t}, \hat{t}_o, \hat{t}_d, \hat{t}_f\) :
-
Non-dimensional times denoting global events related to ASL
- \(T\) :
-
VF inferior–superior extent (thickness)
- \(T_\mathrm{entry}\) :
-
Subglottal channel length
- \(T_\mathrm{exit}\) :
-
Supraglottal channel length
- \(\delta {\varvec{u}}_v\) :
-
Arbitrary virtual displacement
- \(\varvec{u}\) :
-
VF displacement
- \(\varvec{v}\) :
-
Airflow velocity
- \({\varvec{v}}_g\) :
-
Airflow domain grid velocity
- \(v_c\) :
-
Average speed at which interaction line segment recedes
- \(V^f\) :
-
Glottal air tract volume
- \(V^s\) :
-
Volume of space occupied by VF pair
- \(\partial (V^f)\) :
-
Glottal air tract boundary
- \(\partial (V^s)\) :
-
Bounding surfaces of VF pair
- \(W\) :
-
Glottal air tract width
- \(x_\mathrm{is},x_\mathrm{ml},x_\mathrm{ap}\) :
-
Coordinate axes in inferior–superior, medial–lateral, and anterior–posterior directions, respectively
- \({\mathbf X}_\mathrm{MC}\) :
-
Reference location on mid-coronal plane prone to collision
- :
-
Time derivative
- \(\mathrm{d}\) :
-
Differential operator
- \(\nabla \) :
-
Spatial gradient
- \({()}^T\) :
-
Transpose
- \(\langle \cdot \rangle \) :
-
Average taken over cycle time period
- \(:\) :
-
Double contraction between two tensors
- \(\delta _0\) :
-
Linear-elastic length limit of VF mucus layer
- \(\delta _f\) :
-
Rupture length limit of VF mucus layer
- \(\Delta \) :
-
Length of mucus layer
- \(\phi \) :
-
Cohesive energy or surface tension
- \(\phi _\mathrm{aq}\) :
-
Surface tension of water
- \(\phi ^*_\mathrm{aq}\) :
-
Cohesive energy of an equivalent one-sided aqueous ASL
- \(\eta \) :
-
Non-dimensional ratio of VF bulk and ASL adhesive properties
- \(\lambda \) :
-
Normalized interaction line segment length
- \(\mu \) :
-
Dynamic viscosity of air
- \(\nu \) :
-
Poisson’s ratio of VF tissue
- \(\rho _f\) :
-
Density of air
- \(\rho _s\) :
-
Density of VF tissue
- \(\varvec{\sigma }\) :
-
Cauchy stress tensor in VF volume
- \(\sigma _0\) :
-
Cohesive strength of mucus layer
- \(\hat{\sigma }_0\) :
-
Cohesive strength of rat intestinal mucosal surface
- \(\tau _1\) :
-
Viscoelastic relaxation rate
- \(\varvec{\tau }_f\) :
-
Stress tensor within air flow domain
- \(\varvec{\tau }_s\) :
-
Traction on VF surface
- \(\chi \) :
-
Collision state variable
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Acknowledgments
This work was supported by NIDCD Grant 5R01DC008290-04.
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Bhattacharya, P., Siegmund, T. The role of glottal surface adhesion on vocal folds biomechanics. Biomech Model Mechanobiol 14, 283–295 (2015). https://doi.org/10.1007/s10237-014-0603-7
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DOI: https://doi.org/10.1007/s10237-014-0603-7