Relationship between cooperative motion and the confinement length scale in confined colloidal liquids

The confinement length scale, defined as the critical spacing where deviations from bulk behaviors begin, is widely examined with confined molecular and colloidal liquids, yet its origin and relationship to cooperative motion remain an open question. In this work, we examine the correlation between the sizes of cooperatively rearranging regions (CRRs) to the shift of hard-sphere colloidal glass transition in confined domains. We find that the confinement length scale observed in our prior work can be viewed as a regime where CRRs reach a finite size and sets the range for cooperative motion. Additionally, string-like motions within these mobile regions are enhanced at film thicknesses below the confinement length scale and reach maximal at the smallest thickness examined, suggesting an increase in the fragility of confined suspensions. Confined colloidal liquids exhibit an abrupt increase in dynamic heterogeneity at film thickness below the confinement length scale, where out-of-plane string-like motions become prominent.