The addition of some biofillers like lignocellulosic biomass is an effective way to improve polylactic acid (PLA) properties and to extend its range of applications. Lignin, in powder form, has a wide set of applications as filler in composites, such as UV blocker, antioxydant, charring agent in flame retardancy, mechanical reinforcement or surfactant [1]. However, the strong polar nature of lignin, due to the presence of hydroxyl groups, makes it incompatible with the non-polar PLA matrix, resulting in a poor interfacial filler-matrix adhesion. Acetylation and phosphorylation are two lignin chemical modifications used to enhance the mechanical and thermal properties of lignin/PLA composites respectively. These properties are strongly related with the interfacial adhesion between fillers and PLA [2]. To characterize the interfacial adhesion, the determination of lignin and PLA surface energy components is required. Surface energy is an intrinsic property of each liquid and solid material and, for a solid, it is obtained by means of equilibrium contact angle measurements. However, for powders or porous materials more issues arise, notably due to the imbibition of the liquid into the medium. An apparent contact angle can be obtained using wicking tests and the well-known modified Washburn equation for porous materials. However this method, that theoretically allows the determination of apparent advancing contact angles, does not always give reliable results due to the Washburn hypotheses that are not respected or no longer valid. A new simple method using a modified Jurin law for porous media is proposed here in order to determine reliable apparent equilibrium contact angles and obtain surface energy of fillers [3]. An experimental protocol based on capillary wicking (Figure 1) and coupled to the modified Jurin law was set and applied to lignin particles used as reinforcement in PLA. Some treatments were performed on lignin to improve the adhesion with PLA and the modifications due to the treatments were characterized. Reliable surface energy and dispersive and polar components of untreated and treated lignin were obtained revealing the efficiency of the proposed method. Lignins were then used as reinforcements for PLA. The microstructure of the biocomposites was characterized to highlight the modification of adhesion at the filler/matrix interface due to the modification of fillers surface properties [3]. [1] Duval et al., A review on lignin-based polymeric, micro- and nano-structured materials. Reactive & Functional Polymers 85 (2014) 78–96. [2] Guo et al., The influence of compatibility on the structure and properties of PLA/lignin biocomposites by chemical modification. Polymers (2020), 12(1), 56. [3] Carretier et al. An efficient solution to determine surface energy of powders and porous media: application to untreated and treated lignin. Applied Surface Science (2021) https://doi.org/10.1016/j.apsusc.2021.152159.