Experimental Studies Of Drying Colloidal Suspensions

After evaporation,colloidal suspension may deposit a ring pattern and this is called the coffee-ring effect.Because of the coffee-ring effect,the application of colloidal suspension may be affected.For example,homogeneous depositions,instead of coffee-ring pattern,are more desirable in applications such as ink-jet printing,and oil painting.In recent years,the topic of how to suppress the coffee-ring effect has attracted more and more attention.Recent study demonstrated that the coffee-ring effect can be suppressed by changing the shape of the colloidal particle.In this work,the effects of particle softness on the coffee-ring effect are investigated.Using video microscopy,we investigated the drying process of colloidal suspension of hard particle and soft particle respectively.The evolution of the droplet radius,particle dynamics during the evaporation and the resultant deposit of colloids are quantitatively studied.The results demonstrate that the particle softness would not affect the droplet's radius in the drying process.However,two distinct types of drying patterns are observed.One is the ring-pattern for the hard particles due to the coffee-ring effect,and the other one is a uniform pattern for the soft particles where the coffee-ring effect is suppressed.Based on previous work on the drying of suspension made of ellipsoids,we proposed that suppress of coffee-ring effect results from the deformation of soft particle.