Abstract
We consider the general problem of laser pulse heating of a spherical dielectric particle embedded in a liquid. The discussed range of the problem parameters is typical for medical and biological applications. We focus on the case, when the heat diffusivity in the particle is of the same order of magnitude as that in the fluid. We perform quantitative analysis of the heat transfer equation based on interplay of four characteristic scales of the problem, namely the particle radius, the characteristic depth of light absorption in the material of the particle and the two heat diffusion lengths: in the particle and in the embedding liquid. A new quantitative characteristic of the laser action, that is the cooling time, describing the temporal scale of the cooling down of the particle after the laser pulse is over, is introduced and discussed. Simple analytical formulas for the temperature rise in the center of the particle and at its surface as well as for the cooling time are obtained. We show that at the appropriate choice of the problem parameters the cooling time may be by many orders of magnitude larger the laser pulse duration. It makes possible to minimize the undesirable damage of healthy tissues owing to the finite size of the laser beam and scattering of the laser radiation, simultaneously keeping the total hyperthermia period large enough to kill the pathogenic cells. An example of application of the developed approach to optimization of the therapeutic effect at the laser heating of particles for cancer therapy is presented.
Highlights
The problem of the laser pulse heating of absorbing particles embedded in a transparent liquid medium is important for different applications, including stimulation of chemical reactions, laser sintering, selective killing of pathogenic bacteria or cancer cells, etc
In ref. [8] it was shown that dimensional analysis of the non-steady heat diffusion problem based upon the concept of the characteristic scales gives rise to simple formulas for the temperature rise at the surface of the particle Ts
Our analysis results in simple analytical expressions, describing the heating with a laser pulse of a dielectric particle embedded in a transparent liquid and its cooling down after the pulse is over
Summary
The problem of the laser pulse heating of absorbing particles embedded in a transparent liquid medium is important for different applications, including stimulation of chemical reactions, laser sintering, selective killing of pathogenic bacteria or cancer cells, etc. Owing to the quasi-steadiness of the temperature field in this case, the energy balance means that at any moment of time the entire power of the laser beam released in the particle is transferred to the surrounding liquid and by heat diffusion in the liquid the heat flux is conveyed to “infinity”, which plays the role of a sink in this problem.
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