A continuous droplet generator comprising: an outer housing defining a working liquid reservoir configured to maintain a working liquid; at least one heater configured to change the phase of the working liquid; and an evaporation-condensation tube positioned at least partially within the outer housing, wherein the evaporation-condensation tube comprises: a gas inlet configured to receive a particle-containing gas flow; a gas outlet configured to export generated condensed droplets; a wall; an outer wall surface configured to be in contact with the working liquid; and an inner wall surface configured to be in contact with the particle-containing gas flow, wherein the continuous droplet generator is operable to transport vapor to the particle-containing gas flow through the evaporation-condensation tube by diffusion and vapor pressure difference, thereby increasing a relative humidity of the particle-containing gas flow to grow particles within the particle-containing gas flow by nuclei heterogeneous condensation; and wherein at least an inner wall surface of the evaporation-condensation tube is non-wetting; characterized in that the continuous droplet generator comprises a further particle-free gas inlet port operable to introduce a continuous particle-free gas flow coaxially surrounding the particle-containing gas flow within the evaporation-condensation tube, wherein the continuous particle-free gas flow comprises air, nitrogen, or combinations thereof. an outer housing defining a working liquid reservoir configured to maintain a working liquid; at least one heater configured to change the phase of the working liquid; and an evaporation-condensation tube positioned at least partially within the outer housing, wherein the evaporation-condensation tube comprises: a gas inlet configured to receive a particle-containing gas flow; a gas outlet configured to export generated condensed droplets; a wall; an outer wall surface configured to be in contact with the working liquid; and an inner wall surface configured to be in contact with the particle-containing gas flow, a gas inlet configured to receive a particle-containing gas flow; a gas outlet configured to export generated condensed droplets; a wall; an outer wall surface configured to be in contact with the working liquid; and an inner wall surface configured to be in contact with the particle-containing gas flow, wherein the continuous droplet generator is operable to transport vapor to the particle-containing gas flow through the evaporation-condensation tube by diffusion and vapor pressure difference, thereby increasing a relative humidity of the particle-containing gas flow to grow particles within the particle-containing gas flow by nuclei heterogeneous condensation; and wherein at least an inner wall surface of the evaporation-condensation tube is non-wetting; characterized in that the continuous droplet generator comprises a further particle-free gas inlet port operable to introduce a continuous particle-free gas flow coaxially surrounding the particle-containing gas flow within the evaporation-condensation tube, wherein the continuous particle-free gas flow comprises air, nitrogen, or combinations thereof.