대진폭 전단유동장에서 잔탄검 농후계의 비선형 점탄성 거동 연구: 응력파형 및 Lissajous 패턴 해석

Abstract

The objective of the present study is to phenomenologically characterize the nonlinear rheological behavior of concentrated xanthan gum systems in large amplitude oscillatory shear (LAOS) flow fields by means of stress waveform and Lissajous pattern analysis. Using an Advanced Rheometric Expansion System (ARES), the dynamic viscoelastic behavior of aqueous xanthan gum solutions with different concentrations has been experimentally investigated in LAOS flow conditions with a various combination of several fixed strain amplitudes and constant angular frequencies. The main findings obtained from this study are summarized as follows: (1) When a sinusoidal deformation with large strain amplitude is applied, a distorted and nonsinusoidal but symmetrical stress response waveform is observed with time. (2) A saw-tooth shaped stress signal detected at large strain amplitudes may arise from a unique microstructure of xanthan polymer chains. A small peak of stress wave appearing at the position of maximum and minimum stress represents a nonlinear viscous nature of concentrated xanthan gum systems in LAOS flow fields. (3) As an increase in polymer concentration, the shape of stress wave becomes sharper and more distorted. This trend may be explained by an increase in structural density. (4) As a decrease in angular frequency, the stress wave exhibits a more distorted shape and both of the maximum and minimum peaks of a saw-tooth shaped stress response becomes more dominant. (5) At relatively small strain amplitudes, the Lissajous patterns (stress versus strain rate loops) show an elliptical form and their normalized ones are coincident with each other. When larger strain amplitudes are applied, however, the Lissajous patterns are noticeably nonelliptical, and moreover, as the strain amplitude is further increased, the tips of loops become more pointed with exhibiting a characteristic "S" shape.