Experimental Investigations On Interfacial ζ Potential Under The Effect Of Flow Field By Fluorescence Photobleaching Electrochemistry Analyzer

Electric double layer is a fundamental physical phenomenon,which exists widely at the twophase and three-phase interfaces.EDL are of great importance in biomedical,chemical and environmental engineering fields.ζ potential is a key electrical parameter to describe the electric property of EDL.Previous studies on chemically uniform surfaces generally rely on an approximately uniform ζ potential distribution.However,recent studies have shown that flat channel walls display a nonuniform ζ potential under the presence of an external flow field.Unfortunately,limited by existing measurement techniques,it is difficult to make effective and high spatial resolution measurements of the local nonuniform ζ potential,which severely limits the understanding on the fundamental mechanism of EDL from physical perspective.In this investigation,we developed fluorescence photobleaching electrochemistry analyzer(FLEA)technique to measure the local interfacial ζ potential induced by external flow field with high spatial resolution.The influence of inhomogeneous ζ potential on interfacial flow was theoretically studied as well.The primary results of this investigation are as follows:First,we measured ζ potential of chemically uniform coverslip surfaces under a static flow field obtained by FLEA technique.It is found that ζ potential gradually becomes more negative as flow rate Q is increased.Based on nonlinearly fitting,the local ζ potential in stationary flow field is calculated to be-78.7 mV which is consistent to the reports from conventional electrochemical analysis techniques.The validity of FLEA technique for measuring ζ potential is validated.Then,the ζ potential distribution of chemically uniform surface in a microchannel under steady state flow field generated by pressure-driven flow was studied based on FLEA technique.It is found that ζ potential had shown variation at different streamwise and spanwise positions,i.e.,a two-dimensional distribution.It is found that(1)the absolute value of ζ potential gradually decreases from the flow centerline(y=0)to both sidewalls,and(2)ζ potential becomes increasingly negative downstream along the streamwise direction(x direction).In addition,ζ potential gradually becomes more and more negative with increasing pH at the same conductivity σ.It also becomes increasingly negative with increasing conductivity σ at the same pH value.Subsequently,based on aforementioned experimental results,the distribution of electric volume force(Fe)near interface due to the inhomogeneous ζ potential and its effect on the flow were theoretically analyzed.(1)The two-dimensional distribution of ζ potential can lead to a three-dimensional distribution of electric potential near solid-liquid interface,which in turn generates net charges,nonuniform electric volume force(Fe)and its curl(T=▽×Fe)locally.Thus,it is possible to drive fluid to generate counter-rotating streamwise vortex pairs.(2)Under pressure driven basic flow,the counter-rotating streamwise vortex pair generates an approximately symmetric streamwise helical structure near wall surface and gradually climbs upward to form an ascending counter-rotating streamwise vortex pair.Finally,a novel burst mechanism for the coherent structure of turbulent boundary layer is schematically proposed.When a small perturbation is introduced into high-speed flow field,the small flow perturbation induces spatiotemporal fluctuations of ζ potential,which in turn generates initial streamwise vortices and vorticity fluctuations.Subsequently,the streamwise vortices develop downstream,perturbing the flow field,enhancing the velocity fluctuations and the spatiotemporal fluctuations of ζ potential accordingly.Finally,the feedback loop amplifies the strength of streamwise vortices which are sufficiently large to realize the burst of coherent structures(i.e.,hairpin vortices)in turbulent boundary layer.In this investigation,we developed a reliable and high spatial resolution technique for electrochemical analysis.With FLEA technique,we provide a breakthrough experimental observation that verifies the two-dimensional spatial distribution of ζ potential at microscale level for the first time.We hope the current study can significantly promote the development of complex and nonuniform surface materials.Besides,the novel burst mechanism of coherent structures(e.g.,hairpin vortices)in turbulent boundary layers can deepen our understand on the phenomenon from physical perspective.