## 1 Introduction  
Glycerol, a trihydric alcohol, is employed as a heat transfer fluid [1], representing a non-toxic alternative to widely used ethylene glycol [2]. Furthermore, glycerol finds its application in flow-based biosensors as a component of specialized buffer solutions [3]. The abovementioned applications can be used for the organization of a fluid flow. Regarding biosensor systems, biological macromolecules under study are either introduced directly into the flow or immobilized on the sensor surface, which is in direct physical contact with the fluid flow [3; 4; 5; 6; 7]. In this situation, the fluid flow is directly interacting with the biological macromolecules. Regarding bovine serum albumin (BSA), [PERSON] et al. demonstrated that the direct action of a fluid flow on biological macromolecules can induce their aggregation [8]. It should be emphasized that the fluid flow can also act on biological macromolecules indirectly. In other words, the fluid flow can affect biological macromolecules even when there is no direct mechanical contact between the moleculesand the flow. Below, we consider one of the possible mechanisms of the indirect action of the fluid flow on enzymes. The flow of both aqueous [9; 10; 11; 12; 13; 14; 15] and non-aqueous [16; 17; 18; 19; 20; 21] fluids can well cause the triboelectric generation of electric charge [9; 20; 21; 22]. This widely employed phenomenon is the development of the so-called triboelectric generators [11; 12; 13; 14; 18]. It should be emphasized that, despite the fact that aqueous fluids are used for this purpose, in the majority of cases [11; 12; 13; 14], the successful development of oil-employing triboelectric devices was recently reported [18]. [PERSON] et al. have found that, in the case of glycerol, the triboelectric generation of charge is very significant in comparison with the majority of other liquids tested [21]. In this context, it should be borne in mind that triboelectrically induced fields can influence biological macromolecules [15; 16; 17], exhibiting an indirect action of a fluid flow on these macromolecules. Additional attention should also be paid in the case of the triboelectric effect of fluid flow through pipes [9; 21], including pipe coils, which are used as heat exchangers in bioreactors [22]. Heat exchangers are key components of bioreactors operating with enzymes [23; 24]. Since glycerol, which was found to be triboelectrically active [21], is often used as a heat transfer agent [1], further research is required in order to investigate the possible effects of its flow on biological molecules, including enzymes. And this is the aim of our study. The effect of an external triboelectrically induced field on biological macromolecules is often manifested in the form of the aggregation of these macromolecules under the field action [15; 16; 17]. Atomic force microscopy (AFM) represents a powerful method, which allows one to investigate the aggregation of biological macromolecules [25; 26; 27], including enzymes [15; 16; 17; 28; 29; 30; 31; 32; 33; 34] with high (up to single-molecule) resolution. This feature of AFM allows one to even reveal subtle effects, which are indistinguishable by macroscopic methods such as spectrophotometry [29]. AFM is widely employed for studying the aggregation state of various enzymes. In [31], AFM has allowed to determine the ratio between monomers and oligomers of cytochrome CYP102A1, whose oligomers were reported to have a higher enzymatic activity than its monomers [35]. Namely, with the use of an AFM probe with typical (10 nm) tip curvature radius, this ratio was found to be 0.5:0.5. Moreover, the use of a supersharp AFM probe with tip curvature radius of 2 nm has allowed the determination of the ratio between different oligomers of this enzyme (dimers/trimers/tetramers = 0.30:1:0.1). This example clearly illustrates the excellent suitability of AFM for enzyme aggregation studies. Furthermore, [PERSON] et al.