Aim: To study the changes in rheological properties, namely the parameters of the hysteresis loops and yield stress versus time for human semen after ejaculation. Methods: Ejaculates were obtained from volunteers and immediately put into the test cup of a Brookfield Programmable DV-11 Rheometer, by which the hysteresis loops and yield stress were determined. Results: (1) Yield stress values dropped down from more than 3000 mPa to 60 mPa in about 5 minutes after ejaculation; (2) The shape of the hysteresis loops of shear stress versus shear rate was changed from the counter-clockwise direction, that enclosed a large area, into the clockwise direction, that enclosed a very small area. Conclusion: Human ejaculate originally possesses semi-solid or visco-elastic body behavior and in 5 minutes after liquefaction, it becomes a thixotropic fluid or shearing thinning fluid with very low viscosity.
    
    

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