## 4 Discussion  
In our present research, we have studied the influence of glycerol flow, organized in a ground-shielded cylindrical coil, on the properties of horseradish peroxidase enzyme. The adsorption properties of the enzyme have been investigated by AFM, while its enzymatic activity against ABTS substrate has been assessed by spectrophotometry. Previously, in [60], it was noted that both moving and static objects can influence the aggregation state of enzymes. This phenomenon is called the \"[PERSON] effect\" [60]. Particularly, the case with an enzyme incubated above a coil with flowing glycerol [16] was emphasized [60]. According to our results, AFM has allowed us to reveal an increase in the HRP aggregation on mica. Moreover, a slight increase in the enzymatic activity of HRP has been observed after the incubation of the enzyme near the coil. These effects can be explained  
Figure 5: Absorbance at 405 nm vs. time curves recorded for a working HRP sample (incubated at a 2 cm distance from the coil’s side; red curve) and for control HRP sample (incubated three meters away from the experimental setup; blue curve). by the action of an electromagnetic field, which is induced by the flow of glycerol, on the enzyme. The origin of this field can be explained by the occurrence of the so-called triboelectric effect. Below, we consider this phenomenon in more detail. In our experimental setup, the flow of glycerol through a polymeric pipe has been organized. The flow of a liquid is known to be accompanied by friction between the liquid and the pipe surface [61]. And this friction is known to be the driving force of the triboelectric effect, which is also known as \"friction-driven contact electrification\" [62]. In general, this effect occurs in solid-solid [62; 63], liquid-solid [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21], solid-gas [64] and liquid-liquid [65] systems [62]. The motion of the triboelectrically generated charges induces an electromagnetic field. The exact fundamental origin of the triboelectric generation of charge was only recently explained from the viewpoint of thermoelectric physics [62]. Frictional heat was found to be a fundamental factor characterizing triboelectrification [62; 63]. [PERSON] et al. have explained the occurrence of a stronger triboelectric effect in insulating polymeric systems by the combination of their properties, namely their low density and thermal conductivity [62]. Based on the above information, the dielectric characteristics of the materials involved in the triboelectric system represent one of the factors influencing the triboelectric effect. Namely, stronger triboelectric effects were reported for the systems based on dielectric materials, in contrast to weaker ones observed for conducting systems [62; 66]. Additionally, in this connection, we must emphasize that in our reported experiments, both the liquid (glycerol) and the solid (silicone pipe) are dielectrics, thus favoring the occurrence of a strong triboelectric effect. Another important factor is the chemical composition of the triboelectric system materials. With regard to the liquid-solid systems, the triboelectric effect was reported to be dependent on the content of chemical groups in the liquid--apart from the dielectric properties of the materials involved [21]. Namely, in their very recent work, [PERSON] et al. found that in liquid-solid systems, hydroxyl groups enhance the triboelectric effect, while hydrocarbon groups suppress it [21]. Among the liquids studied by these authors, the strongest effect was observed for 70% sorbitol solution and glycerol (though the sorbitol solution contained water). It is to be noted that pure hydrocarbons (such as hexane) were not studied [21]. Nevertheless, the results reported by [PERSON] et al. revealed one more factor explaining strong triboelectric effects observed with glycerol. It is to be emphasized that in our experiments reported herein, the coil with flowing glycerol has been ground-shielded.