Reduce temperature of fluid inside an isolated system without dissipating heat Suppose you have a perfectly isolated system filled with a fluid, which cannot transfer heat nor matter to the external environment. 
Suppose you want to reduce the temperature of the fluid within this isolated system.
Is it possible to reduce the fluid temperature without dissipating heat to the external environment?
How can I transfer the energy of the fluid to any other energy form (electrical, mechanical, etc.) so that to reduce the temperature within the isolated system?
 A: The thermodynamic definition of an isolated system is that it cannot exchange mass nor any form of energy with its surroundings. The two possible forms of energy transfer are heat and work. 
You have specifically ruled out heat transfer and mass transfer between your system and surrounding. You did not specifically rule out work transfer. However, since you are calling the system isolated, by the thermodynamic definition of isolated no form of work transfer can occur. 
Conclusion: On the basis of you stipulating that the system is isolated, there can be no change in temperature of the liquid.

Thanks, it helped. So if we suppose the system not isolated (but we
  keep excluding mass and heat transfers) we can reduce the fluid
  temperature by converting heat into work, using an heat engine right?
  But still I think it is not possible to have 100 percent of heat
  converted into work. This raises another question: can I make this
  work transfer without transferring also heat from the system to the
  environment? Anyway, thanks for the reply

You can't convert heat into work because you've already specified that no heat can be transferred into or out of the system (fluid). 
On the other hand if the fluid was an "insulated" system (cannot transfer heat), as opposed to an isolated system (cannot transfer heat or work) there may be the possibility that the fluid can due expansion work on the surroundings, by expanding the walls of its container against external pressure. Then there is the possibility of a temperature decrease per the first law $\Delta U=Q-W$, where $Q$=0 and $W$ is positive expansion work done by the fluid at the expense of internal energy. 
However, most fluids are relatively incompressible, that is, it is difficult to compress or expand the fluid. That means the work done by the expansion of the fluid would be very little, resulting in only a small decrease in the fluid temperature. But a decrease nonetheless. 
Hope this helps. 
