Effect of surfactants on the stability of thin liquid film flow on a rotating disk.
The effect of surfactants on surface instabilities of thin liquid film flow on a rotating disk was studied at different flow rates Q (0.5<or=Q<or=2 ml/s), rotation rates f (33<or=f<or=100 Hz), and volume concentrations rho(s) of the surface active protein BSA (10(-4)<or=rho(s)<or=4 x 10(-4) wt%) in the far field of large radii r (11<or=r<or=25 cm). With these parameters, ranges of the Reynolds number Re (0.48<or=Re<or=4.3), Weber number We (22.3<or=We<or=1410), and surface elasticity numbers zeta or E (0.02<or=zeta<or=62; 0.1<or=E<or=1.4) were covered. The ratio Re/Re(crit) varied between values <<1 and 15000. Surface waves were induced with a constant gas jet directed onto the rotating liquid film in order to investigate their propagation velocities. Measured data are in fairly good agreement with linear and nonlinear theories. The damping effect of surfactants was studied using self-excited waves. It could be shown that the adsorbing monolayer is not in thermodynamic equilibrium at small radii r, but approaches equilibrium at larger r. Assuming the validity of linear stability theory with respect to wave damping, the adsorption kinetics of BSA on the flowing film could be traced. The determination of the transition radius from wavy to smooth film surfaces provides a tool to estimate local film pressures in situ.
Leneweit
Gero
G
TU Darmstadt, Fachgebiet Strömungslehre und Aerodynamik, Petersenstr 30, D-64287 Darmstadt, Germany. leneweit.carus@t-online.de
Roesner
Karl G
KG
Koehler
Reinhard
R