Colloidal Particles and Their Counterions (IMAGE) Princeton University, Engineering School Caption Princeton chemical engineers accurately represented colloidal systems using a maximum of four colloidal particles, shown here in blue, and their oppositely charged counterparts, called counterions, shown here in red. Credit Athanassios Panagiotopoulos Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.