Deposition of metallic colloids under sub-cooled nucleate boiling

Heterogeneous nucleate boiling causes the deposition of colloidal particles and governs the growth rate of porous deposit layers on heating surfaces when dilute metallic colloidal suspensions are present in aqueous medium. Post-experimental analysis reveals that these deposits are found in the form of spots of variable thickness around the bubble nucleation site. It is shown here that thickness of these spots and deposit amount is directly related to evaporation rate which is dependent on heating wall temperature and fluid temperature. The experimental observations suggest that the deposition occurs around the contact line of bubble and extends underneath area of bubble micro-layer. Rate of transportation of particles to the wall is directly related to frequency of bubble nucleation and departure diameter of departing bubbles. These transported particles deposit onto the heating surface with an attachment probability, which is modeled here as a function of colloid–surface interaction potential. A model of deposition rate has been developed on the basis of evaporative flux Eq″, attachment probability patt and bulk concentration of colloidal particles C0. This model provides qualitative explanation for experimental results and suggests that deposition flux has linear dependence on the quantity pattC0Eq″.

Deposit pattern of nano-particles for different boiling conditions (Jakob number) Ja.Figure optionsDownload as PowerPoint slideHighlights
► Colloidal deposition under boiling is dependent upon evaporation flux.
► Deposit size and thickness are correlated with bubble dynamics.
► Linear dependence on bulk concentration and attachment probability.