the endoplasmic reticulum (ER) membrane where neutral lipids are synthesized. While the molecular mechanisms of this process are unknown, it appears that LD biogenesis is following a threestage process: neutral lipid synthesis, lens formation (via intra-membrane lipid accumulation), and finally LD formation. [3] At relatively low concentrations, neutral lipids accumulate between the two leaflets of the ER bilayer to eventually form a lipid reservoir with a lens shape. [3,4] Numerical modeling suggests that such lenses could exist in the ER membrane with a size of tens of nanometers. [3,5,6] Upon the continuous production of neutral lipids, they accumulate into the reservoir which leads to a lipid lens growth. Above a certain size, this lipid lens becomes unstable and can lead to a spontaneous budding of the oily reservoir. [3,7] Ultimately, the droplet pinchoff from the membrane leads to its release into the cytosol. [8][9][10] LDs dynamically adapt to metabolic changes in the cell and balance uptake of free fatty acids by their esterification and storage as triglycerides, and consumption of triglycerides by the release of fatty acids under catabolic conditions. These essential metabolic functions of LDs are executed by proteins on the LD surface, many of which dynamically partition from the ER bilayer to the LD monolayer membrane. [11,12] Aberrant LD functions are implicated in numerous metabolic diseases such as obesity, diabetes,The ORCID identification number(s) for the author(s) of this article can be found under

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Lipid droplets (LD) are organelles localized in the membrane of the endoplasmic reticulum (ER) that play an important role in many biological functions. Free LDs that have been released from the ER membrane and are present in the cytosol resemble an oil-in-water emulsion. The surface of an LD is coated with a phospholipid monolayer, and the core of an LD is composed of neutral lipids. Adipose differentiation-related protein (ADRP), also known as perilipin-2, is a protein that surrounds the LD, together with the phospholipid monolayer. ADRP molecules are involved in assisting in the storage of neutral lipids within LDs. In this article, we focus our interest on the influence of ADRP molecules on the 3D shape of bilayer-embedded LDs and the diffusion of phospholipids in the monolayer covering LDs. For this study, we employed two different microfluidic setups: one to produce and explore bilayer-embedded LDs and a second one to mimic the surface of a single LD. Using the first setup, we demonstrate that ADRP molecules stay preferentially localized on the surfaces of bilayer-embedded LDs, and we study their 3D-shape in the presence of ADRP. Using the second setup, we performed FRAP experiments to measure the phospholipid diffusion on a model LD surface as a function of the ADRP concentration. Although the presence of proteins on the LD surface minimally affects the phospholipid and protein motility, ADRP appears to have a significant effect on the 3D structure of LDs embedded in the bilayer.