This paper investigates the effects of density differences on turbulent mixing layers between two streams of different gases, particularly nitrogen and helium. The authors used a novel experimental setup to study plane turbulent mixing, focusing on the large coherent structures that dominate the mixing layer. High-speed movies and shadowgraphs revealed that these structures convect at nearly constant speed and increase in size and spacing discontinuously through amalgamation with neighboring structures. Density fluctuations were measured, suggesting that turbulent mixing and entrainment are processes of entanglement on the scale of these large structures. The study found that significant changes in the density ratio across the mixing layer have a relatively small effect on the spreading angle, indicating that the strong effects observed when one stream is supersonic are due to compressibility effects rather than density effects. The paper also discusses the implications of these findings for understanding the dynamics of turbulent mixing layers in both subsonic and supersonic flows.This paper investigates the effects of density differences on turbulent mixing layers between two streams of different gases, particularly nitrogen and helium. The authors used a novel experimental setup to study plane turbulent mixing, focusing on the large coherent structures that dominate the mixing layer. High-speed movies and shadowgraphs revealed that these structures convect at nearly constant speed and increase in size and spacing discontinuously through amalgamation with neighboring structures. Density fluctuations were measured, suggesting that turbulent mixing and entrainment are processes of entanglement on the scale of these large structures. The study found that significant changes in the density ratio across the mixing layer have a relatively small effect on the spreading angle, indicating that the strong effects observed when one stream is supersonic are due to compressibility effects rather than density effects. The paper also discusses the implications of these findings for understanding the dynamics of turbulent mixing layers in both subsonic and supersonic flows.