Lacunarity as image descriptor for the evaluation of the dispersion degree of carbonaceous substrates in lubricants

One of the main problems related to the aging of lubricants is the formation and accumulation of carbonaceous deposits inside the matrix. Consequently, the development of additives for the dispersion of these agglomerates is mandatory to increase the performances and life of the lubricant. Usually, the dispersant performances are evaluated by means of expensive and time-consuming engine tests. Here a new lab-scale method based on optical microscopy for the direct assessment of the dispersing capacity of a lubricant formulation is presented. The degree of dispersion of the investigated samples is calculated directly on optical micrographs using the lacunarity descriptor which is defined as gappiness or heterogeneity associated to an image. We demonstrate that low lacunarity values correspond to well dispersed systems while the presence of aggregates is responsible for an increase of micrograph lacunarity. The method was validated on reference oils and different commercial carbonaceous substrates [3] showing optimal reproducibility and sensitivity. The obtained results in terms of dispersant ability were coupled with the study of the colloidal behavior of the dispersants in minimal and/or complete formulations establishing a structure- properties relation to explain the different dispersant capacity.