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In this work,the rheological changes of several crude oil samples exposed to ultrasonic waves for different time intervals in addition to the effect of temperature on viscosity behavior of heavy crude oils were investigated using a series of steady shear flow and oscillatory tests.The colloidal structural evolutions of flocs in oil samples were illustrated by analysis of the size distribution of flocculated asphaltene particles (confocal microscopy tests).The rheological investigations indicate that the ultrasonic irradiation dissolved heavy components in crude oil.After ultrasonic treatment,the Kouh-e-Mond crude oil was found to be pseudoplastic.In addition,confocal microscopy confirms that there was an optimum duration for ultrasonic irradiation,at which the viscosity and flocculation rate of asphaltenic crude oils reduced to the minimum values.The optimum was found to be approximately 40 min for the Kouh-e-Mond crude oil.Experimental results illustrate that the ultrasonic irradiation could disaggregate heavy colloid components in crude oil,and breakdown of asphaltene molecules would only occur in a specific time interval of irradiation.Also according to the temperature sweep test,the oil temperature rise caused by ultrasonic irradiation was not the main reason for rheological changes of the crude oil and this alteration may be due to physical and chemical phenomena induced by sonication in crude oil.