nanofluids , proposed firstly by choi , are dispersions of nanoparticles ( metals , oxides , nitrides and carbon materials etc . ) in base fluids ( water , ethylene glycol and engine oil etc . ) . they have wide ranges of applications , e.g. , to enhance heat transfer [ 1 - 11 ] , as magnetic nanofluids   and other special applications [ 13 - 18 ] . however , long - term stability of nanofluids is a major concern for the engineering applications . therefore , technique breakthroughs are needed to produce well - dispersed and long - term stable nanofluids . use of functionalized nanoparticles is a promising approach to achieve long - term stability of nanofluid . functionalization of silica nanoparticles has been reported earlier in literature [ 19 - 24 ] . the current authors have reported earlier on effects of nanoparticles in nanofluid on thermal performance in water - cuo system . the present work reports on the synthesis of functionalized silica ( sio2 ) nanoparticles by grafting silanes directly to the surface of silica nanoparticles in original nanoparticle solutions . one was a commercial nanoparticle solution , ludox tm-40 , purchased from aldrich company in usa . the ludox tm-40 ( with the silica nanoparticle mass concentration of 40% in water , the average diameter of nanoparticles of about 30 nm ) contains naoh , surfactants and so on to stabilize the silica nanoparticles in the water solution . the other was a self - made nanoparticle solution prepared by the two - step method . silica nanoparticle powders were firstly dispersed into deionized water , and the suspension was then oscillated in an ultrasonic bath for 12 h. silica nanoparticle powders with an average diameter of about 30 nm were commercial products of gas condensation . two silanes of 2-[methoxy ( polyethyleneoxy ) propyl ] trimethoxysilane ( mppts ) and ( 3-glycidoxylproyl ) trimethyoxysilane ( gps ) were received from gelest company in usa and used without further purification . the synthesis methods using two silanes are described as follows : schematic of the functionalization ( 1 ) mppts - functionalized nanoparticles mppts - functionalized nanoparticles were synthesized simply by grafting directly a certain amount of mppts to silica nanoparticles in silica solutions . as an example , 25 g of ludox tm-40 was diluted with 50 g deionized water . to the silica suspension , 1.45 g of mppts was added dropwise under vigorous stirring at room temperature . generally , the mass ratio ( defined as the ratio of the mass of the silane to that of the silica nanoparticles in silica solutions ) ranges from 0.1 to 1.0 . the entire mixture was heated to 70c and stirred and oscillated vigorously for 48 h at that temperature . the obtained solution was then dialyzed in the snakeskin dialysis tubing ( 3.5 k mwco , purchased from pierce ) with deionized water as the dialyzate for 48 h and thereafter freeze - dried to give white fine nanoparticle powders . gps - functionalized nanoparticles were synthesized in a similar manner . generally , the mass ratio ranges from 0.1 to 1.0 . as an example , 1.35 g of gps was diluted with 10 g of deionized water , and the ph value of the solution was adjusted to 2 with dilute hcl . after vigorous stirring for 4 h at room temperature , its ph value was adjusted to 10 with dilute naoh . meanwhile , 25 g of ludox tm-40 was diluted with 40 g of deionized water . the obtained gps solution was then added dropwise to the silica suspension under vigorous stirring and oscillation and kept at 70c for 48 h in an ultrasonic bath . the obtained solution was then purified by dialysis and freeze - dried in the same manner shown earlier . to investigate the reaction degree of the silica and the silane , tga instrument ( ta q5000 ) was used to analyze how many silanes are grafted on the silica nanoparticles . the fine powders of functionalized nanoparticles were burnt at 700c in a tga instrument , and the organic components on the surfaces of the functionalized nanoparticles were burnt away . the tga instrument can record the mass loss of the functionalized nanoparticles during the whole process . table 1 shows the mass loss percentage caused by the organic components for functionalized nanoparticles at different mass ratio . the mass ratio is the ratio of the mass of the reacting silane to that of the reacting silica . the mass loss is the mass loss percentage of the functionalized silica before and after the tga process . results show that the mass loss increases with the increase in the reacting mass ratio . more silanes can be grafted on the silica nanoparticles at larger mass ratios . at the same mass ratio , the nanoparticles functionalized with mppts have a larger mass loss compared to those with gps . mass loss of functionalized nanoparticles after the tga process mass ratio represents the mass ratio of the reacting silane and silica ; mass loss represents the mass loss percentage caused by the organic components after the functionalized nanoparticles are burnt by the tga instrument after the functionalization process , nanofluids were prepared by the two - step method using functionalized nanoparticles and deionized water . functionalized nanoparticles were dispersed into deionized water , and the solution was thereafter vigorously stirred and oscillated in an ultrasonic bath for 12 h at room temperature ( the solution can also be standing for 12 h with the environmental temperature of 50c ) . functionalized nanoparticles can still keep dispersing well after the nanofluid has been standing for 6 months and no sedimentation was observed . the dispersing state of functionalized nanoparticles after 6 months is still under investigation and will be reported later . figure 2a and 2b are the transmisson electron microscope ( tem ) pictures of nanofluids consisting of nanoparticles before and after the functionalizing process ( prepared with ludox tm-40 and mppts , the mass ratio is 0.123 ) , respectively . figure 2c and 2d are the tem pictures of nanofluids consisting of nanoparticles before and after the functionalizing process ( prepared with self - made nanoparticle solution and gps , the mass ratio is 0.115 ) , respectively . as is shown , functionalized nanoparticles have no aggregation and can disperse well . in the present study , the mass concentration of the functionalized nanofluid ( nanofluids consisting of functionalized nanoparticles ) prepared with commercial silica solution can go beyond 50% , and the other functionalized nanofluid prepared with self - made silica solution can reach maximally 10% under the condition that no aggregation occurs . tem pictures of nanofluids consisting of nanoparticles before and after the functionalizing process a nanoparticles in ludox tm-40 b functionalized nanoparticles prepared with ludox tm-40 and mppts c nanoparticles in self - made nanoparticle solution d functionalized nanoparticles prepared with self - made nanoparticle solution and gps dynamic light scattering ( dls ) measurement of functionalized nanoparticles was taken using a malvern zetasizer nano particle characterization system . diameter distribution of functionalized nanoparticles ( prepared with self - made and gps , the mass ratio is 0.115 ) is indicated in fig . the average diameter of the functionalized nanoparticles is about 30 nm , which is in consistent with the result shown in fig . 2 . diameter distribution of functionalized nanoparticles an applicable prospect of functionalized nanofluids is in the phase - change heat transfer . up to now , almost all phase - change heat transfer studies of conventional nanofluids ( consisting of nanoparticles without functionalization ) reported that nanoparticles fouled on the heated surface after the boiling process . a thin porous layer formed on the heated surface and affected strongly the boiling characteristics . in order to investigate the heat transfer characteristics of functionalized nanofluids in the phase - change heat transfer , a pool boiling experiment using both the conventional nanofluid and the functionalized nanofluid was carried out on a smooth copper surface positioned horizontally . the experimental system and the experiment was carried out at atmospheric pressure with the fluid temperature of about 100c . the copper surface was kept heating for 3 h with the heat flux of about 500 kw / m . figure 4a shows the sem picture of the heated surface before the boiling experiment . figure 4b and 4c show the sem pictures of the heated surface after the boiling experiment using both the functionalized nanofluid ( consisting of functionalized nanoparticles prepared with self - made nanoparticle solution and gps , the mass ratio is 0.115 ) and the original self - made nanofluid . it was found that no porous fouling layer formed on the heated surface after boiling process using the functionalized nanofluid . the nanoparticles can be easily flushed away by water . however , a fouling layer of nanoparticles forms on the heated surface after the boiling experiment using the original self - made silica solution . the evaporation of nanofluids with subsequent settlement of unfunctionalized nanoparticles initially contained in it could be the reason for the formation of this layer . it is not the case , however , for functionalized nanoparticles since their improved solubility . sem pictures of the heated surface during the boiling experiment a before the boiling experiment b using the functionalized nanofluid c using the conventional nanofluid since no fouling layer exists after the boiling process , the functionalized nanofluid can be applicable in heat pipes that utilize the phase - change heat transfer of working fluids to dissipate heat . this is quite helpful for the long - term stable running of heat pipes since the heated surfaces keep the same and the nanofluid keep dispersing well . further experiment is undergoing for better understanding the application of functionalized nanofluids in heat pipes . in addition , the well - dispersed and stable functionalized nanofluid can be applicable to microchannels in electronic devices and compact cooling modules to dissipate heat more effectively . it is also useful for enhancing the mass transfer , such as the oxygen absorption in chemical and biochemical industry , ammonia absorption in the refrigeration technique . overbeek ( dlvo ) theory of colloid stability is commonly used to describe the particle substrate interfacial interactions in the base fluid . in this theory , both an attractive force and a repulsive force existing between particles the attractive force , also called the van der waals force , includes attractions between atoms , molecules and surfaces . the repulsive force arises from the overlap of electrical double layers that appear on the surfaces of particles when they are placed into the base fluid . the overlap of their double layers interrupts their own electrostatic stability and the electrostatic repulsion causes the repulsive force , when two particles get too close together . particles in the base fluid can keep stable if the repulsive force is stronger than the attractive force . another theory named as the steric stabilization effect is also a key explanation for the stability of particles in the base fluid . the steric stabilization effect arises from the fact that polymers gathering on the surface of nanoparticles occupy a certain amount of space . if nanoparticles are brought too close together , the space is compressed . an associated repulsive force helps separate nanoparticles from each other and restrains the aggregation of nanoparticles . adding ions to the nanofluid according to the dlvo theory can enhance the electrostatic repulsion of nanoparticles . adding surfactants to the nanofluid is helpful for the steric stabilization effect . these are ways how ludox tm-40 is treated to help the silica nanoparticles stabilize in the water solution . in ludox also , the steric effect by surfactants may be broken down when the nanofluid is heated or the brownian motion of nanoparticles is extremely strong . a better way , as is indicated in this paper , is to graft polymers onto the surface of nanoparticles . the grafting is achieved with the covalent bonding , which is quite stable and not easy to be broken down . the grafting silanes on nanoparticles have the steric stabilization effect to stabilize nanoparticles . the covalent bonding o  si  is stable chemical bonding to facilitate the application of functionalized nanoparticles regardless of the working conditions , like the heated case . besides , to achieve a better and larger solubility of nanoparticles in water , silanes containing polar structures are chosen . due to the solubility rule of similarity , the polar structure grafted on the surface of the silica nanoparticles increases the solubility of functionalized nanoparticles in water ( which is also a polar substance ) . the nanofluids with functionalized nanoparticles gain the merits of good dispersing , stable bonding , no surfactants needed and large solubility . similar methods may be used to other nanoparticles for heat transfer applications . the functionalized nanofluid made by commercial product of ludox tm-40 however , using the functionalized nanofluid prepared by the cheap self - made silica solution is a more economic way for industrial application due to its cheap price and large producing capacity . to investigate the reaction degree of the silica and the silane , tga instrument ( ta q5000 ) was used to analyze how many silanes are grafted on the silica nanoparticles . the fine powders of functionalized nanoparticles were burnt at 700c in a tga instrument , and the organic components on the surfaces of the functionalized nanoparticles were burnt away . the tga instrument can record the mass loss of the functionalized nanoparticles during the whole process . table 1 shows the mass loss percentage caused by the organic components for functionalized nanoparticles at different mass ratio . the mass ratio is the ratio of the mass of the reacting silane to that of the reacting silica . the mass loss is the mass loss percentage of the functionalized silica before and after the tga process . results show that the mass loss increases with the increase in the reacting mass ratio . more silanes can be grafted on the silica nanoparticles at larger mass ratios . at the same mass ratio , the nanoparticles functionalized with mppts have a larger mass loss compared to those with gps . mass loss of functionalized nanoparticles after the tga process mass ratio represents the mass ratio of the reacting silane and silica ; mass loss represents the mass loss percentage caused by the organic components after the functionalized nanoparticles are burnt by the tga instrument after the functionalization process , nanofluids were prepared by the two - step method using functionalized nanoparticles and deionized water . functionalized nanoparticles were dispersed into deionized water , and the solution was thereafter vigorously stirred and oscillated in an ultrasonic bath for 12 h at room temperature ( the solution can also be standing for 12 h with the environmental temperature of 50c ) . functionalized nanoparticles can still keep dispersing well after the nanofluid has been standing for 6 months and no sedimentation was observed . the dispersing state of functionalized nanoparticles after 6 months is still under investigation and will be reported later . figure 2a and 2b are the transmisson electron microscope ( tem ) pictures of nanofluids consisting of nanoparticles before and after the functionalizing process ( prepared with ludox tm-40 and mppts , the mass ratio is 0.123 ) , respectively . figure 2c and 2d are the tem pictures of nanofluids consisting of nanoparticles before and after the functionalizing process ( prepared with self - made nanoparticle solution and gps , the mass ratio is 0.115 ) , respectively . as is shown , functionalized nanoparticles have no aggregation and can disperse well . in the present study , the mass concentration of the functionalized nanofluid ( nanofluids consisting of functionalized nanoparticles ) prepared with commercial silica solution can go beyond 50% , and the other functionalized nanofluid prepared with self - made silica solution can reach maximally 10% under the condition that no aggregation occurs . tem pictures of nanofluids consisting of nanoparticles before and after the functionalizing process a nanoparticles in ludox tm-40 b functionalized nanoparticles prepared with ludox tm-40 and mppts c nanoparticles in self - made nanoparticle solution d functionalized nanoparticles prepared with self - made nanoparticle solution and gps dynamic light scattering ( dls ) measurement of functionalized nanoparticles was taken using a malvern zetasizer nano particle characterization system . diameter distribution of functionalized nanoparticles ( prepared with self - made and gps , the mass ratio is 0.115 ) is indicated in fig . the average diameter of the functionalized nanoparticles is about 30 nm , which is in consistent with the result shown in fig . an applicable prospect of functionalized nanofluids is in the phase - change heat transfer . up to now , almost all phase - change heat transfer studies of conventional nanofluids ( consisting of nanoparticles without functionalization ) reported that nanoparticles fouled on the heated surface after the boiling process . a thin porous layer formed on the heated surface and affected strongly the boiling characteristics . in order to investigate the heat transfer characteristics of functionalized nanofluids in the phase - change heat transfer , a pool boiling experiment using both the conventional nanofluid and the functionalized nanofluid was carried out on a smooth copper surface positioned horizontally . the experiment was carried out at atmospheric pressure with the fluid temperature of about 100c . the copper surface was kept heating for 3 h with the heat flux of about 500 kw / m . figure 4a shows the sem picture of the heated surface before the boiling experiment . figure 4b and 4c show the sem pictures of the heated surface after the boiling experiment using both the functionalized nanofluid ( consisting of functionalized nanoparticles prepared with self - made nanoparticle solution and gps , the mass ratio is 0.115 ) and the original self - made nanofluid . it was found that no porous fouling layer formed on the heated surface after boiling process using the functionalized nanofluid . the nanoparticles can be easily flushed away by water . however , a fouling layer of nanoparticles forms on the heated surface after the boiling experiment using the original self - made silica solution . the evaporation of nanofluids with subsequent settlement of unfunctionalized nanoparticles initially contained in it could be the reason for the formation of this layer . it is not the case , however , for functionalized nanoparticles since their improved solubility . sem pictures of the heated surface during the boiling experiment a before the boiling experiment b using the functionalized nanofluid c using the conventional nanofluid since no fouling layer exists after the boiling process , the functionalized nanofluid can be applicable in heat pipes that utilize the phase - change heat transfer of working fluids to dissipate heat . this is quite helpful for the long - term stable running of heat pipes since the heated surfaces keep the same and the nanofluid keep dispersing well . further experiment is undergoing for better understanding the application of functionalized nanofluids in heat pipes . in addition , the well - dispersed and stable functionalized nanofluid can be applicable to microchannels in electronic devices and compact cooling modules to dissipate heat more effectively . it is also useful for enhancing the mass transfer , such as the oxygen absorption in chemical and biochemical industry , ammonia absorption in the refrigeration technique . the derjaguin  landau  verwey  overbeek ( dlvo ) theory of colloid stability is commonly used to describe the particle substrate interfacial interactions in the base fluid . in this theory , both an attractive force and a repulsive force existing between particles the attractive force , also called the van der waals force , includes attractions between atoms , molecules and surfaces . the repulsive force arises from the overlap of electrical double layers that appear on the surfaces of particles when they are placed into the base fluid . the overlap of their double layers interrupts their own electrostatic stability and the electrostatic repulsion causes the repulsive force , when two particles get too close together . particles in the base fluid can keep stable if the repulsive force is stronger than the attractive force . another theory named as the steric stabilization effect is also a key explanation for the stability of particles in the base fluid . the steric stabilization effect arises from the fact that polymers gathering on the surface of nanoparticles occupy a certain amount of space . an associated repulsive force helps separate nanoparticles from each other and restrains the aggregation of nanoparticles . adding ions to the nanofluid according to the dlvo theory can enhance the electrostatic repulsion of nanoparticles . adding surfactants to the nanofluid is helpful for the steric stabilization effect . these are ways how ludox tm-40 is treated to help the silica nanoparticles stabilize in the water solution . in ludox tm-40 , anions ( like oh ) are used to stabilize silica nanoparticles . but nanoparticles may be applied in acidic or neutral conditions . also , the steric effect by surfactants may be broken down when the nanofluid is heated or the brownian motion of nanoparticles is extremely strong . a better way , as is indicated in this paper , is to graft polymers onto the surface of nanoparticles . the grafting is achieved with the covalent bonding , which is quite stable and not easy to be broken down . the grafting silanes on nanoparticles have the steric stabilization effect to stabilize nanoparticles . the covalent bonding o  si  is stable chemical bonding to facilitate the application of functionalized nanoparticles regardless of the working conditions , like the heated case . besides , to achieve a better and larger solubility of nanoparticles in water , the polar structure grafted on the surface of the silica nanoparticles increases the solubility of functionalized nanoparticles in water ( which is also a polar substance ) . the nanofluids with functionalized nanoparticles gain the merits of good dispersing , stable bonding , no surfactants needed and large solubility . the functionalized nanofluid made by commercial product of ludox tm-40 can prepare the nanofluid with a higher nanoparticle mass concentration . however , using the functionalized nanofluid prepared by the cheap self - made silica solution is a more economic way for industrial application due to its cheap price and large producing capacity . a method of surface functionalization of silica nanoparticles to prepare a kind of functionalized nanofluid was presented . the nanofluid with functionalized nanoparticles can keep long - term stability and very good dispersing . functionalized nanoparticles can still keep dispersing well after the nanofluid has been standing for 6 months and no sedimentation is found . one unique characteristic of the functionalized nanofluids is that no porous sediment layer forms on the heated surface after boiling process . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
a method of surface functionalization of silica nanoparticles was used to prepare a kind of stable nanofluid . 
 the functionalization was achieved by grafting silanes directly to the surface of silica nanoparticles in silica solutions ( both a commercial solution and a self - made silica solution were used ) . 
 the functionalized nanoparticles were used to make nanofluids , in which well - dispersed nanoparticles can keep good stability . 
 one of the unique characteristics of the nanofluids is that no deposition layer forms on the heated surface after a pool boiling process . 
 the nanofluids have applicable prospect in thermal engineering fields with the phase - change heat transfer .