## 3 Results  
### AFM Measurements  
Figure 2 displays typical AFM images of mica substrate surface with HRP macromolecules, adsorbed from the analyzed solutions. In working experiments, the HRP solution was incubated in the experimental setup near either the inflow or the outflow section of the pipe. In control experiments, the protein solution was incubated far away (at a 10 m distance) from the setup.  
In the typical AFM image, obtained in the control experiment (Figure 2a), one can clearly distinguish compact 1- to 1.2-nm-high objects. These objects can be attributed to HRP macromolecules, since no such objects was detected on the surface of a mica substrate incubated in a protein-free buffer.  
In case of working experiments (Figure 2b,c), after the incubation of HRP solution near either the inflow or the outflow section of the pipe, compact-shaped objects are also observed, similar to the case with control experiments.  
Figure 3 displays the distributions of AFM-visualized objects with height (density function plots) \(\rho\)(\(h\)) (Figure 3a) and the histogram representing the absolute number of visualized AFM-visualized objects, normalized per 400 \(\upmu\)m\({}^{2}\) area (Figure 3b).  
The curves shown in Figure 3a indicate that in control experiments, the maximum of the density function corresponds to 1.0 \(\pm\) 0.2 nm. The molecular weight \(M_{r}\) of HRP is 40 to 44 kDa [12,13]. According to the previously reported data, AFM images of proteins of similar molecular weight also have a height \(h_{max}\) of about 1 nm (putiaredoxin reductase, \(h_{max}\) = 1.8 nm [27], \(M_{r}\) = 45.6 kDa [28]; adrenodoxin reductase, \(h_{max}\) = 1.8 nm [29], \(M_{r}\) = 54 kDa [30]). For this consideration, the 1-nm-high objects, visualized on mica surface, can be attributed to monomeric HRP.  
As regards the working experiments, when the protein solution was incubated near the inflow linear section of the pipe, the characteristics of the density function were similar to those obtained in control experiments. On the contrary, in the case of incubation near the outflow linear section of the pipe, the maximum of the density function plot shifted to the right (\(h_{max}\)\(\approx\) 1.3 nm, Figure 3a, red line), and the contribution of the right distribution wing increased. That is, an appearance of objects with increased heights, which can be attributed to protein aggregates, was observed in the latter case.  
Figure 3: Results of processing of AFM data obtained for HRP solutions. (**a**) Typical plots of distribution of the imaged objects with height \(\rho(h)\). (**b**) Absolute number of AFM-visualized particles, normalized per 400 \(\upmu\)m\({}^{2}\). The cell with HRP solution was placed far away at a 10-m distance from the experimental setup (control experiment, black line and bars), near the inflow (blue line and bars) or near the outflow (red line and bars) section of the pipe.  
Figure 2: Typical AFM (atomic force microscopy) images of the mica substrate surface with adsorbed HRP (horseradish peroxidase) macromolecules and cross-sections, corresponding to the lines in the AFM images, obtained in control experiments (**a**), and when the HRP solution was incubated near either inflow (**b**) or outflow (**c**) sections of the pipe.  
As shown in Figure 3b, the absolute number of visualized objects (\(N_{400}\)), obtained in our experiments, varies considerably. Thus, in the control experiment, \(N_{400}\) amounted to 4867 objects per 400 \(\upmu\)m\({}^{2}\). When the HRP solution was incubated near either the inflow or the outflow section of the flow-based system, \(N_{400}\) amounted to 851 and 7780 objects per 400 \(\upmu\)m\({}^{2}\), respectively. That is, the number of mica-adsorbed HRP macromolecules, observed in working experiments, significantly differs from that obtained in control experiments.