Sains Ma1aysiana 28: 101-118 (1999)                                                                             Sains Fizis dan Gunaan/

                                                                                                                                            Physical and Applied Science


Negative Surface Viscosities in Fluid-Fluid Interfaces

with Adsorbed Insoluble Surfactants


D.M.A. Buzza

Department of Physics, Astronomy and Polymer

I.RC. University of Leeds

Leeds LS2 9JT United Kingdom





Recent measurements of the surface viscoelasticity of liquid surfaces with adsorbed surfactant using surface light scattering have yielded negative dilational viscosities (ε’ < 0). In this paper, we present a physical explanation for this phenomenon by constructing a molecularly based theory for the interfacial viscoelasticity of an insoluble surfactant monolayer. Using the interfacial viscoelasticity, we then calculate the resultant surface light scattering from the interface. Our microscopic theory predicts the presence of two additional surface elastic constants neglected in current treatments, namely a bending modulus, κ and a coupling constant λ. We show that for thick monolayers, the coupling constant λ can lead to apparent negative values of ε’. This is the first time that this result has been explained for insoluble monolayers using a physically realistic model.




Pengukuran kelikatkenyalan permukaan yang terkini terhadap permukaan cecair dengan surfaktant terjerap melalui penyerakan cahaya telah menunjukkan kelikatan dilatasi negatif (ε’ < 0). Kertas kerja ini memberikan pemerihalan fizis fenomena ini dengan membina satu teori berasaskan molekul bagi kelikatkenyalan antaramuka dalam satu surfaktant monolapisan tak larut. Dengan menggunakan kelikatkenyalan antaramuka ini, hasil serakan cahaya permukaan dari antaramuka ini telah dikira. Teori mikroskopik ini menjangka kewujudan dua pemalar kekenyalan permukaan yang tidak diambilkira dalam pengiran semasa iaitu modulus membengkok κ dan pemalar gandingan λ. Dalam kertas ini ditunjukkan bahawa untuk monolapisan yang tebal, pemalar gandingan boleh memberikan nilai ε’ yang nampak negatif. Kertas ini memberikan pemerihalan pertama untuk monolapisan tidak larut dengan menggunakan model fizik yang realistik.





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