The effect of contact angle hysteresis on adroplet in a viscoelastic two-phase system

• We investigate the dynamic behaviour of a two-dimensional (2D) droplet adhering to a wall in Poiseuille flow at low Reynolds numbers, in a system where either the droplet is viscoelastic (V/N) or the surrounding medium (N/V). The fluid viscoelasticity has been modeled by the Giesekus constitutive equation, and the Cahn–Hilliard Phase-Field method is used to capture the interface between two phases. The contact angle hysteresis is represented by an advancing contact angle and a receding contact angle . The results reveal that the deformation of the viscoelastic drop over time is changed due to the presence of polymeric molecules, and it can be categorized in two stages prior to depinning of the contact lines. In the first stage, the viscoelastic droplet speeds up and deforms faster, while in the second stage, the Newtonian counterpart accelerates and its deformation outpaces the viscoelastic droplet. The deformation of viscoelastic drop is retarded significantly in the second stage with increasing Deborah number De. In the V/N case, the viscous bending is enhanced on the receding side for small De, but it is weakened by further increase in De, and this non-monotonic behavior brings about an increase in the receding contact line velocity at small De and a decrease at large De. On the advancing side, the viscous bending is decreased monotonically, and hence the advancing contact line velocity is decreased with increasing De. The non-monotonic behavior on the receding side is attributed to the emergence of outward pulling stresses in the vicinity of the receding contact line and the inception of strain-hardening at higher De, while the reduction in the viscous bending at the advancing side is the result of just strain-hardening due to the presence of dominant extensional flow on the advancing side. Finally, in the N/V system, the viscoelasticity of the medium suppresses the droplet deformation on both receding and advancing sides, and this effect is more pronounced with increasing De; the weakening effect of viscous bending is enhanced significantly at the advancing side by increasing the Giesekus mobility parameter in the N/V system. These results give a thorough understanding of viscoelastic effect on both drop deformation and depinning of both contact lines over a surface with contact angle hysteresis.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Strömningsmekanik och akustik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Fluid Mechanics and Acoustics (hsv//eng)

Nyckelord

Wetting

viscoelasticity

contact angle hysteresis

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