US 9,981,240 Bl  30  29  5.  The device of claim  2,  wherein said solvent  comprises  one or more compounds  selected from the  group consisting  of water, dimethyl  formamide, dimethylsulfoxide,  isopropa› nol, acetone,  tetrahydrofuran,  and  combinations thereof.  6.  The device of claim  1,  wherein said pH-control  sub- stance is triggered  with  electromagnetic  radiation to gener› ate said acid or  said base.  7.  The device of claim  6,  wherein said pH-control  sub› stance is a photoacid  generator or  a  photobase generator.  8.  The device of claim  1,  wherein said pH-control  sub- stance is triggered  with a change in  temperature  to generate  said acid or said  base.  9.  The device of claim  1,  wherein said trigger  source is  configured  to  uniformly trigger  said pH-control substance  in  space within said  reaction microfluidic  region.  10.  The device of claim  1,  wherein said trigger  source is  configured  to trigger said  pH-control substance  in a spatial  pattern within said  reaction microfluidic  region.  11.  The device of claim  1,  wherein said trigger  source is  disposed in electrical  communication  with a controller  to  maintain pH within  said reaction microfluidic  region within  a  preselected pH range.  .  12.  Th~ device of claim  1,  wherein said trigger  source is  disposed m electrical  communication  with a controller  to  oscillate pH within  said reaction microfluidic  region.  13.  The device of claim  1,  wherein said trigger  source is  disposed in electrical  communication  with a controller  to  maintain pH rate  of change within  said reaction microfluidic  region below a preselected  maximum pH rate  of change.  14.  The device of claim  1,  wherein said reaction  micro- fluidic region contains  one or more particle  arrays.  15.  The device of claim  1,  wherein said device  further  comprises  a  particle-array outlet  port disposed in  fluid  communication  with said reaction  microfluidic region.  16.  The device of  claim  15,  wherein said device  is  configured for continuous  operation.  17.  ~~e  device of claim  1,  said device further  comprising  an add1t10nal reaction  microfluidic region  disposed in fluid  communication  with said reaction  microfluidic region.  18.  A method of using  the device of claim  1,  said method  comprising:  *  *  *  *  *  _2~.  The method of claim  24,  wherein said titrated  pH is  within 0.5 pH units  of said average isoelectric  point of said  particles.  24.  ~he ~ethod of claim  18,  wherein said titrating  pH  :esults 1~ a t1~rated  pH within 1.5 pH  units of said average  1soelectnc pomt  of said particles.  23.  The method of  claim  18,  wherein said particles  contain a core and  a shell, wherein  said core comprises  a  metal, a polymer,  a cermnic, or a  combination thereof  and  wherein said shell  comprises an oxide,  a fluoride, a sulfide,  or a  combination thereof.  22.  The method of claim  18,  wherein said particles  are  characterized  by an average  particle size from  about 1  nanometer  to about 100 microns.  21.  The method of claim  20,  wherein said particle  array is  recovered from said  reaction microfluidic  region.  20.  The method of  claim  18,  wherein said particles  assemble into a  particle array.  19.  The method of claim  18,  wherein said first  and second  fluids are flowed  simultaneously  into said  droplet-generating  microfluidic region.  titrating pH within  said reaction microfluidic  region  towards the average  isoelectric point  of said particles.  triggering generation  of an acid or a  base from said  pH-control substance,  within said reaction  microfluidic  region; and  flowing said droplets  into said reaction  microfluidic  region;  flowing said second  fluid through said  second-fluid inlet  port, into said  droplet-generating  microfluidic region,  th~reby generating  a plurality of droplets  containing  said first fluid, wherein  said droplets are  phase-sepa› rated from said second  fluid;  flo~ing s~id first  fluid through said  first-fluid inlet port,  mto said  droplet-generating  microfluidic region•  pr~pari~g _or o?tain)ng  a second fluid that  is  substantially  1mm1sc1ble  m said first fluid;  preparing_ or obtaining  a first fluid  comprising  a plurality  of particles to be  assembled,  a solvent, and said  pH› control substance; 